We have already had occasion to offer a few observations on the production of Mules [...] We have at this place to notice a similar reproduction between two specie of a more prominent character on the great theatre of life, between, in fact, the two tyrants of the world - the African Lion and the Asiatic Tiger. [...]

But to return to our Lion-Tiger cubs. Mr Atkins, an itinerant exhibitor, and dealer himself, bred the Lion, the father of these cubs. He was a very fine and very valuable beast, for his beauty and the docility of his disposition, the ferocity of which had never been entirely developed by natural habits. At the period in question, he was about four years old. The Tigress, the mother of the cubs, is supposed to be about four or five years old. She has been in Mr Atkins's possession for about two years, and was, probably, taken very young, as the gentleness of her disposition seems to evince.

These two animals, ever since the arrival of the Tigress, have been confined in one den, and have always agreed well together. From the beginning of their being so placed, there had been frequent possibility of issue, though the first, consisting of a litter of three cubs, was not born till the 17th of October, 1824, the result of a more particular intercourse, which lasted ten or twelve days, in the beginning of the previous July. They were born at Windsor, and were shortly afterwards honoured by a visit from his Majesty. The Lion, unfortunately, died about six weeks afterwards. The cubs were taken from the Tigress immediately after birth, and were fostered by several bitches and a Goat; they are all alive, and promise, at present, to attain maturity.

In regard to their personal appearance, we feel constrained, after what has already been said, to be very brief in our description, an omission, however, we hope to compensate by the figures. These show them at three months old, and we have added figures of the young, both of Lions and Tigers, that they may be compared with this singular breed.

The young of the Lion are calculated to deceive an inexperienced observer, from the fact of their being striped transversely, so as to induce the opinion, at first sight, that they belong to the Tiger, and, in this respect, the cubs in question agreed with those of the Lion. In the young Lions, however, these stripes soon become obliterated, but in those before us, they appear to be getting more decided and permanent, and, in fact, to be assuming the permanent Tigrine character. Our Mules, in common with ordinary Lions, were born without any traces of a mane, or of a tuft at the end of the tail. Their fur, in general, was rather woolly; the external ear was pendant toward the extremity; the nails were constantly out, and not cased in the sheath; and, in these particulars, they agreed with the common cubs of Lions. Their colour was dirty-yellow or blanket-colour; but from the nose over the head, along the back, and upper side of the tail, the colour was much darker, and, on these parts, the transverse stripes were stronger, and the forehead was covered with obscure spots, slighter indications of which appeared also on other parts of the body. The shape of the head, as appears by the figures, is assimilated to that of the father (the Lion); the superficies, of the body, on the other hand, is like that of the Tigress.

(Note: "Mule" referred to any hybrid, not just to equine hybrids)

HYBRIDITY IN ANIMALS,
CONSIDERED IN REFERENCE TO THE QUESTION OF THE UNITY OF THE HUMAN SPECIES
By Samuel George Morton, M.D. (1846)

The American Journal of Science, Volume 53
(Read before the Academy of Nat. Sci. of Philadelphia, Nov. 4 and 11, 1846.)
[In receiving this paper, we commit ourselves (as in other cases) to none of the opinions of the author. We may add, that we have no fear of discussion on any point in science. Facts are the markings of a Divine hand around and within us, and when studied in all their bearings, they lead in the end to the establishment of Truth.-Eds.]

PART I. INTRODUCTORY REMARKS.

THE facts connected with hybridity in the inferior classes of animals, have an important bearing on one of the most interesting questions in Ethnography; and it is in reference to this question, that we now propose to arrange and review them. [Dr. James Cowles Prichard, the first Ethnographer of this or any age, has, with great care and candor, collected many of the following examples of hybridity, although to my view, they conflict strongly with his main position.—See Researches into the Physical History of Mankind, vol. i.]

It was taught by Buffon, John Hunter, and other naturalists of the past century, and is yet assumed by some learned men of the present day, that the hybrid offspring of two distinct species of animals, are incapable of reproducing their kind; thus making hybridity the test of specific character. It follows, according to this supposed law of nature, that if mankind embraced several species, the intermixture of these would go no further than to produce a sterile hybrid variety. But since all the races are capable of producing, with each other, a progeny more or less fertile, it is inferred that they must all belong to one and the same species. This is the question at issue.

It may, at first view, appear superfluous to go over the whole ground of inquiry; but apart from its Ethnographic relations, it is my wish to call attention to a branch of science that has hitherto been singularly neglected, and perhaps more so than any other. Having sought in vain for some collective exposition of its details, I was at length induced to examine them for myself; and in now giving them publicity, I respectfully solicit, from practical observers, any authenticated examples of an analogous kind, that may not be embraced in this memoir. We shall merely further premise that naturalists have differed as to the import of the word species, but we know of no better definition than that which is expressed by “separate origin and distinctness of race, evinced by the constant transmission of some characteristic peculiarity of organization.” The term race has been indefinitely and conveniently used in those instances in which it is difficult to decide whether an individual of any tribe of plants or animals, is a distinct species, or only a variety of some other species. Races are properly successions of individuals propagated from any given stock; and we agree with the learned Dr. Prichard, from whom we cite these definitions, that when races can be proved to possess certain primordial distinctions, which have been transmitted unbroken, they should be regarded as true species.*
[* Researches into the Physical History of Mankind, 3d ed., p. 105, 100.—For some highly interesting views of this question, and their practical application, see Prof. Haldeman's Enumeration of Fresh Water Mollusca, in Boston Jour. of Nat. Hist., 1844. Further researches into Ethnographic affinities, may render it probable that what are now termed the five races of men, would be more appropriately called groups; that each of these groups is again divisible into a greater or smaller number of primary races, reason of which has expanded from an aboriginal nucleus or centre. Thus I conceive that there were several centres for the American group of races, of which the highest in the scale are the Toltecan nations, the lowest the Fuegians. Nor does this view conflict with the general principle, that all these nations and tribes have had, as I have elsewhere expressed it, a common origin; inasmuch as by this term is only meant an indigenous relation to the country they inhabit, and that collective identity of physical traits, mental and moral endowments, language, &c., which characterize all the American races. The same remarks are applicable to all the other human races; but in the present infant state of Ethnographic science, the designation of these primitive centres, is a task of equal delicacy and difficulty. I may here observe, that whenever I have ventured an opinion on this question, it has been in favor of the doctrine of primeval diversities among then,_an original adaptation of the several races to those varied circumstances of climate and locality, which, while congenial to the one are destructive to the other; and subsequent investigations have confirmed me in these views. See Crania Americana, p. 3; Crania AEgyptiaca, p. 37; and Distinctive Characteristics of the Aboriginal Race of America, p. 36.]
Let us now proceed to examine the question before us, commencing with the larger mammiferous animals, and proceeding from these to birds, fishes, insects and plants.

EQUINE HYBRIDS.

The common mule, the progeny of the ass and mare, has been familiar to man since the days of Homer; and it is equally well known that with this animal, the hybrid born, as a general rule, begins and terminates. But the result appears to depend much on temperature; for in the south of Spain, mules have often been observed to produce young; and M. de la Malle observes that this phenomenon is frequent in hot climates, in which their period of gestation is twelve months, being the same as that of the mare. The same author quotes from Cohumella, the remark of Mago, a Carthagenian agriculturalist, that in his country the fecundity of the mule was a frequent event, although it was regarded as a prodigy in Greece and Italy. He adds, that these mixed mules do not cross again with each other, but only with the primitive species that has given them birth. [* M. de la Malle, Ann. des Sciences Nat., xxvii, p. 235.]

The ancients gave the name ginnus to the offspring of the mule with the mare, which appears to have been a common animal among the Romans, who called it also the little mule, (parvum mulum [M. de la Malle, ibid., p. 143.]) Prevost and Dumas, repeating the experiments of Lieuenhoeck, assure us that the sterility of these mules in northern climates, depends on an absence of spermatic animalcules; but the latter must be present in hot countries, to explain the phenomena of reproduction. The Hinny, on the other hand, is the offspring of the horse and a female ass—Bardo er equo et asina—an animal of a very refractory disposition, and little esteemed, either in ancient or modern times; nor have I been able to obtain any facts in relation to its reproductiveness. Again, when a male, derived from the cross “between the she ass and the male onager, (Equus onager,) is allowed to couple with the mare, the offspring is more docile than either parent, and unites the beauty of form and gentle disposition of the father with the strength and swiftness of his grandsire; [Columella, quoted ut supra, p. 135. ] whence the ancients preferred the onager to the ass for the production of mules, and Mr. Gliddon informs me this opinion is prevalent in Egypt, at the present day. The Baron Cuvier informs us that he had seen the cross between the ass and zebra, and between the female zebra and horse. [Regne animal, i, p. 182 ]

The ass is not the proximate species of the genus Equus, when compared with the horse; but that place is held, as Cuvier remarks, by the dziggetai of Asia, (Eq. hemionus.) And two distinguished naturalists, Mr. Bell and Mr. Gray, are even disposed to remove the ass to a separate genus. Without passing judgment on this question, I will merely observe, that in order to obtain a prolific breed of hybrid horses, the true horse should be coupled with the hemionus, under the same adaptation of climate and domesticity that have been bestowed on some other mixed animals; nor until this experiment has been fairly tried, can we speak with absolute certainty of the extent of productiveness of equine mules.

The phenomenon of productiveness has little or no limit among the true horses; whence it has been inferred that they all belong to one species; and that their various forms and colors are solely owing to the diversified circumstances in which they have been placed. But the researches of Hamilton Smith, have not only given rise to much doubt on this subject, but have adduced a surprising array of facts in favor of the opposite opinion.

We must refer to his learned and elaborate essay for the mass of evidence therein embodied; merely observing, on the present occasion, that he separates the horses into five primitive stocks, which appear to constitute “distinct, though osculating species, or at least races separated at so remote a period, that they claim to have been divided from the earliest times of our present Zoology.” [Natural History of the Equidae, p. 154.]

He adds, that some of these forms yet exist in the wild state on the table-lands of Central Asia, and that all of them were so constituted as to be fusible into a common, specific, but very variable stock, for the purposes of man; and he finally concludes, that if man had been necessitated to cultivate the zebras of South Africa, instead of the horses of Asia, he would have succeeded in amalgamating the three or four known species into one domestic animal, little inferior to the horse itself. [Ibid, p. 75, 183. Fossil remains of the horse, and especially the teeth, have been of late years abundantly found in Europe and Asia, and in North and South America; (and especially near Natchez, by Dr. Dickerson;) showing that this animal was once indigenous to all these widely extended regions; and yet there are now no horses in the Western Hemisphere, excepting those that have descended from the European stock. The indigenous species must have become extinct from some remote and extended cataclysm. It is curious, also, to note that these fossil horses were different in species from the present variety, although they were closely allied to it. There were in ancient times several, perhaps many, species of the genus Equus.]

It therefore becomes a reasonable supposition that some varieties of the horse now known to us, may be hybrid mixtures of proximate species; more especially, since the facts collected by Hamilton Smith, De Azara, and De la Malle, show conclusively that all the domestic horses were reclaimed from an original wild State.

BOVINE HYBRIDS.

In the argument in question, the ox tribe has always been referred to as one of the strongest evidences of the operation of local causes in producing varieties of breed. But the parent type or stock is wholly unknown to naturalists; and although it corresponds in its osteological structure with a fossil species (Bos urus) found throughout Europe, it is extremely doubtful whether all the modifications now familiar to man are derived from this animal. “An opinion has lately been started,” observes a learned zoologist, “that the haunched varieties of cattle, are derived from a different species; against which no conclusive objection can well be made, when it is considered that the Gayal (Bos gavacus) produces a mixed race with the domestic animal; and that the yak of Tartary, (Bos grunniens,) and even the American bison, are equally reported to mix with that species, notwithstanding their anatomical differences, and that the times of gestation are not similar.” [* Griffith's Cuvier, iv, p. 419.-Prichard's Researches, i, p. 190.]

The hybrid offspring of the buffalo and the common breed of cattle, is now familiar in the western parts of the United States, particularly in Missouri and Kentucky; but I have not been informed whether they have ever bred away among themselves, or with either of the parent stocks. I have instituted inquiries on this subject, the results of which I hope to add as a sequel to this memoir. In fact, it is now conceded that all the species of the genus Bos are similarly circumstanced [Loudon's Magazine of Nat. Hist., ix, p. 511] whence we have no difficulty in supposing that among the ox tribe, as among various other classes of animals, hybridity has more or less modified their forms during the long lapse of thousands of years.

BOVINE AND CERVINE HYBRID

The Baron Larrey incidentally mentions in his memoirs, the following circumstance that occurred during his residence at the Bay de Croc, in Newfoundland: “The Carabou (Cervus Wapiti) sometimes comes near the houses. In the night, one of them broke into our sheep-fold, where we had a cow, that became pregnant by him. She no doubt produced a mongrel; but I lost the opportunity of ascertaining the fact, because she was taken back to Brest.” [Memoirs of Military Surgery, &c., Dr. Hall's trans., vol. i., p. 11.]

I see no reason to question any part of this statement, which ceases to astonish us when we regard the many analogous phenomena that are now fully authenticated, and among others the following very remarkable one.

BOVINE AND OVINE HYBRID.

In the article on hybridity, in Brande's Dictionary of Literature and Science, [Article: Hybrid] it is mentioned, without doubt or reservation, that a mule has been obtained between the bull and sheep; a statement that claims our entire credence, from the circumstance that the physiological part of the work in which it is contained, is from the pen of Prof. Owen, of the Royal College of Surgeons.

CERVINE HYBRIDS.

The only example of this class that I have met with in authors, is that between the Indian buck of the Axine species, (Cervus axis,) with the Porcine species, thus giving rise to the well known intermediate stock, called the Spotted Hog-deer. [Hamilton Smith, Equidae, p. 341.]

CAPRINE HYBRIDS

The goat called the wild agagrus, which is found in all the Alpine regions of Europe and Asia, appears in every instance to be a prolific hybrid between the domestic goat and the local wild one of the country it inhabits, although the latter animal may be the ibex, the caucasica, or any other species. [Hamilton Smith, Equidae, p. 341.] A mixed breed has also been obtained between the chamois (Antilope rupicapra) and the common goat. [Idem.]

Ovine Hybrids.

It was until lately supposed by most zoologists, that the domestic sheep, and the Asiatic and American Argali, were mere varieties of one species; but they are now known to be distinct, and are severally designated by the names of Ovis musmon, O. ammon and O. pygargus. The common sheep, called in the systems, O. aries, is generally classed as a variety of the first named species; but recent investigations render it more than probable that several wild species have commingled to form the numerous cultivated races.[ Idem, p. 70.—Blyth, Proceedings of the Zoolog. Soc. of London, 1840.]

This view is to a certain degree sustained by a hybrid product recently obtained in Paris, between a wild moufflon (species not mentioned) and the common sheep; but as this result dates with the present year, the productive faculty of the intermediate animal had not been tested. [Chevreul, Journal des Savants, Juin, 1846, p. 357.]

OVINE AND CAPRINE HYBRIDS.

The ancients, more especially the Romans, regarded all the varieties of domestic sheep as a mixed offspring of the sheep and goat, (Capra hircus.) The possibility of this union was proved by Prof. Pallas, by personal observation during his travels in Russia; and although a doubt has been here and there expressed with respect to it, the fact is now conceded by all naturalists from abundant evidence. Some new and very interesting information has lately been afforded us from another quarter. “For a very long time,” observes M. Chevreul, “an extensive commerce has been carried on in Chili, in the skins of sheep with rather coarse wool, derived from across between the male of the common goat and the ewe, which was obtained as follows: a single goat was placed with six eves, and male hybrids were obtained with a hairy fleece, which was little esteemed for the particular purpose for which it was designed. But by coupling these male hybrids with ewes, the latter were fruitful, and their offspring bore a fine, soft fleece, which is highly valued in the manufacture of shabraques, called also pellians, in Chili. After several generations, the hair becomes coarse and hard, when it becomes necessary to recur to a male hybrid of a former generation, in order to obtain the requisite cross for the production of the perfect fleece. [Journal des Savants, Juin, 1846, p. 357.]

I have only to add, on the same authority, that Prof. Flourens, of Paris, has recently obtained a cross between the wild ram [Moufflon. The particular species is not designated. ] (Ovis musmon) and the female of the common goat.

CERVINE AND OVINE HYBRID.

Hellenius, quoted by Rudolphi, mentions the very interesting case of a Sardinian doe that refused the goat, but was crossed by a ram. The young had the figure of the father, but in color more resembled the mother. These hybrids were again crossed by a Finland ram, and after a few generations assumed the characters of the Finland breed of sheep. [Rudolphi, Beyträge zur Anthropologie, &c., p. 165. ]

CAMELINE HYBRIDS.

The two species of camel, C. bactrianus and C. dromedarius, produce with each other an intermediate offspring, which is said to be fertile without limit. Buffon could not deny this proverbial fact; and in order to obviate a difficulty that conflicted with a favorite opinion, he assumed that these animals must be mere varieties of a single species. Modern science, however, has established, beyond question, the specific differences of the camel and the dromedary. [Cuvier, Regne animal, i, p. 187.]

CANINE HYBRIDS.

If we could admit that all the dogs, with their varied external forms and peculiar instincts, have been derived from a single pair of these animals, we could have no difficulty, I conceive, in adopting so much of Lamarck's theory as relates to the progressive transmutation of species, resulting from what he calls the force of external circumstances; and it is curious to observe, that he especially adduces the canine race in support of his hypothesis. “In nature we seek in vain for mastiffs, harriers, spaniels, greyhounds, and other races between which the differences are so great that they would be readily admitted as specific among wild animals; yet all these have sprung originally from a single race, at first approaching very near to a wolf; if, indeed, the wolf be not the true type which at Some period or other was domesticated by man. [See Lyell's Principles of Geology, B. III, chap. 1, &c.]

He further maintains that the peculiar instincts and functions of animals, the dogs for example, have not resulted from a previous and pre-adapted organization; but that these instincts, on the contrary, have developed by constant use those very organs of which they are the seat. The greyhound for example, has derived his long and slender legs, and his proverbial speed, from the mere habit of running with celerity in pursuing some animals and in escaping from others. The mastiff again has become large, strong and muscular, from habitually seizing and holding animals larger and stronger than himself. In fine, Lamarck applies the same principle to all organized beings, which according to his doctrine have been developed by the mere force of circumstances, a tendency to progressive advancement from the simplest to the most perfect forms. And here we may inquire, if education and domesticity can so vary not only the instincts but the very proportions of anatomical structure in dogs, do we not realize in the theory of Lamarck, a law of nature which would with equal readiness explain the unlimited transmutation of species into each other ?

But is it proved that all the domestic dogs are really derived from a single species : Here again we appeal to one of the latest and best authorities on this question—Charles Hamilton Smith, whose laborious researches have led him to the following conclusions:—that the parents of our domestic dogs are derived from several distinct species, which were constituted with faculties to intermix, and thus to produce the interminable varieties familiar to man; that five of these types belong to the old world alone, viz. the wolf, the buansu, the anthus, the dingo and the jackal ; that a dhole or a thus may have been the progenitor of the grayhound; and that the origin of the primitive mastiff may yet be traced to a lost or undiscovered species belonging to the hyena tribe. [Natural History of the Dog, in Naturalist's Library, voli, p. 104, et passim. The Canis rematica of Burchell, connects the dog with the hyena almost without an interval.]

The wolf, the dog, the jackal and the fox, all intermix with each other. So does the common jackal with the jackal of Senegal. Do they therefore belong to one species? It is well known that the cross between the dog and the European wolf in the experiments of Buffon, did not extend beyond the fourth generation; but the distinguished writer whom we have just quoted, has observed, that the animals were in a state of neglect and restraint, and gradually tended to sterility from their small number, and from the want of recrossings from one or other of the parental stocks. It is worthy of remark, that the dingo of Australia when placed in similar circumstances with the common dog, also becomes sterile in the fourth generation; whence, according to this test, the dingo is not a true dog, but some other species of the genus Canis.

The greatest number of mammae in the common dog is ten, the smallest number, six; in the wild species they are always in pairs, and they never vary in a species. “To what other cause, then, can we ascribe the anomaly in domestic dogs, so justly as to an intermixture of species?” [Ibid, ii, p. 79.]

The dogs that have become wild in Paraguay, always hunt in packs, thus resuming the wolf-like instinct of their progenitors. Will it be said that this is a newly developed instinct; or is it not rather an old one that new wants have reproduced.

It is therefore certain that dissimilar species of the dog tribe are capable of producing a fertile hybrid offspring; and if it was the interest of man again to cultivate and extend these mixed species, there is every probability that the race would become unlimited.

“Experiments show,” observes Mr. Lyell, “that after repeated failures, the union of two recognized species may at last, under very favorable circumstances, give birth to a fertile progeny; and such circumstances,” he adds, “the naturalist may conceive to have occurred again and again in the course of a great lapse of ages.” [Principles of Geology, Book III, chap. 2.]

Every one who is in the least degree acquainted with the natural history of dogs, knows that certain remarkable changes of color, and sometimes of form, take place in particular localities. These changes are usually attributed solely to climate, food, training, and other exterior agents. I do not deny the modifying action of such agents in these and other cases; but it is a reasonable subject of inquiry, whether there may not be something in these localities that favors an effort of nature to reproduce a primitive type : The localities to which we allude, [See Dr. Prichard's Natural History of Man, for an admirable exposition of these and all other facts on which the analogical argument is sounded.] do not operate equally on all varieties of the dog tribe , which we might suppose would be the case if all the canine breeds were derived from a single stock or species. It is important in connection with this subject, to observe that all the pure Indian dogs of North America are of one variety, with erect ears, a wolfish aspect, and having a howl in place of a bark. Most naturalists agree in considering it a reclaimed wolf. The late Mr. Thomas Say, regarded it as the canis latrans or howling wolf, in a state of domestication. It is remarkable, when unmixed, for the uniformity of its characters, which are the same in every locality over thousands of miles in extent. [Carver's Travels in North America, p. 417. See also the plates of the magnificent Atlas of the Prince de Wied’s Travels in this country.] No varieties have arisen from it, excepting by crossing the breed with other dogs, when a hybrid is produced that is prolific without end.

It is much to be regretted that so little is known of the history of the indigenous dogs of America, a subject that affords a fine field for scientific inquiry. While engaged in writing this memoir, I am assured by my friend Dr. M'Coy, an intelligent physician and naturalist, that in the interior of Pennsylvania, the common wolf, C. lupus, has been taken when young, and successfully trained to deer hunting. The difficulty, however, with these animals was, that they devoured the game, unless the sportsman was on the spot to prevent them. To obviate this fault, these wolves were crossed by the common dog : giving rise to a mixed breed, that combined the keener instinct of the wolf with the greater docility of the dog. Should these hybrids reproduce among themselves, or with either of the parental sources, how completely will the history of these animals illustrate the origin of the dog tribe, its primitive domestication, the crosses between different species, and the varieties that must have followed from such intermixture ? I hope yet to be able to lay before the reader all the facts of this singular history.

SURINE HYBRIDS.

Another domestic animal which presents remarkable varieties of form as well as of marking, is the hog ; and these have also been attributed to a single species modified by immemorial domestication. Some new light, however, has recently been thrown on this branch of zoology by Mr. Eyton of London, who has compared the skeletons of the Chinese, the African, [The Sus aethiopicus has even been removed to a separate genus by Cuvier— Phascochaeres. See P. AEliani in Ruppell, Atlas zu der Reise in Nord-Afrika, p.61.] and the English pig, and finds that while they agree in the number of cervical vertebrae, (as indeed all quadrupeds do,) there is a remarkable difference in each of the other classes of these bones. We have not space for details, except to observe that the dorsal vertebrae vary from thirteen to fifteen, the lumbar from four to six, and the caudal from thirteen to twenty. Now, as far as time and circumstances had allowed the experiment to proceed, these several animals bred freely with each other, and in the instance of the Chinese pig, the offspring is unquestionably fruitful. Mr. Eyton very justly remarks, that the above three pigs must be considered as distinct species, or osteological characters can no longer be received as criteria of species; and Hamilton Smith has arrived at the conclusion, that there were three if not four original species, endued with powers of unlimited reproduction.

FELINE HYBRIDS.

These animals, at least the domestic varieties, had long been regarded as of one species; but modern researches have established that the blue or Chartreuse cat, originally belonged to a distinct feline group; the Bengal cat of Pennant pertains to a second ; while the tortoise-shell cat is believed to have sprung from a third group originally indigenous to South America. [Hamilton Smith, Equidae, p. 339.] I believe all these animals produce with each other a fertile offspring. It may be denied, however, that they belong to different species; but that the domestic cat was once of at least two species, seems now decided by the observation of Dr. Ruppell, who finds the embalmed cat of the Egyptians to correspond to the Felis maniculata [Atlas zu der Reise im Nordlichen Afrika, p. 4, tab. I. Prof. Bell has also decided that the Felis catus, found wild in the forests of Europe is different from both the domestic species] of Nubia, and not to the Felis domestica. Where then is the race of cats once so abundant in ancient Egypt : They have probably come down to us so blended with other species that their identity is lost.

De Azara states, in the forests of Paraguay the Felis yaguarundi and the F. eyra, both unite with the domestic cat; and he adds, that should these wild species become in time extirpated, and the mixed breed alone remain, the latter would be very naturally referred with all its varieties, to a single original species. [Quadrupeds of Paraguay, i, p. 174.]

Mixed breeds have also been obtained between the black leopard and the African species, and between the lion and the tigress. The latter cross which is much the more remarkable, produced three cubs, which were doing well at the time the facts were published. [Vide Griffith's Cuvier, ii, p. 448. 1827.] We regret that no further particulars have come under our notice.

FELINE AND MUSTELINE HYBRID.

A most remarkable instance of hybridity between the cat and an animal of a totally distinct genus, is described in the following account, which is published in several of the best scientific periodicals, and appears to be well authenticated. “A domestic cat disappeared from a house in Penza. After being absent some time, she returned; and within the regular time, produced four young ones, two of which strongly resembled the marten. Their claws were not retractile, as in the cat; and the snout was elongated, like that of the pine marten, (Mustela martes.) The two others of the same litter more nearly resembled the cat; as they had retractile claws and round heads. All of them had the black feet, tail and ears of the marten; and they killed birds and small animals more for the pleasure of destroying them than for food. The proprietor endeavored to multiply this race, and to prevent their intermixing with the domestic cats, in which he proved highly successful. In the space of a few years he reared more than a hundred of these animals. A specimen presented to the Imperial Society of Natural History of Moscow, was of the third or fourth generation, and it retained all the characters of the first.” [Loudon's Mag. of Natural History, ix, p. 616. Griffith's Cuvier, ii, p. 489.]

Professor Pallas has described and figured the Perxsa cat, which has long been suspected for a hybrid, although very prolific. It may yet prove to be the animal we have just described.

LEPINE HYBRID.

Amoretti, quoted by Rudolphi, has published the history of a cross between the European or English rabbit, Lepus cuniculus, and the hare, L. timidus. [Rudolphi, Beyträge zur Anthropologie, etc., p. 165. Econd SERIEs, Vol. III, No. 7.-Jan., 1847. ]

PHOCINE HYBRID

Finally, among mammiferous animals, it remains to notice the singular fact discovered by the traveler Steller, and mentioned by Rudolphi, that the sea-lion, Phoca jubata, of Behring's island, produces young with the sea-bear, P. ursina. “I have no doubt of this fact,” adds Prof. Rudolphi, “since Pallas speaks of Rudolphi with the greatest respect, and Telesius proved the accuracy of his observations.” [Ibid. loco citat. Prichard’s Researches. i. p. 142]

Samuel George Morton's essay was in relation to "The Question of the Unity of the Human Species". This cutting from "The Examiner" in 1855 illustrates the line of thinking being pursued. Today it is considered racism masquerading as science. In the mid-1850s some scientists tried to justify a belief that there were different species of humans, with European whites being a superior species of human.

[DISCUSSION OF HYBRIDS IN] HEREDITARY DISEASES OF SHEEP AND PIGS. (Concluded from page 189.) ADDENDUM
Farmer's Gazette and Journal of Practical Horticulture, 10th May 1856

Much difference of opinion prevails as to the relative influence of the male and female parent in determining the characters of the progeny. According to very prevalent notion, the male bestows all valuable qualities, whether of form or of vigour; while the female is regarded merely as passive instrument which hatches, as it were, the male seed—an absurd doctrine long preserved from well-merited obloquy as a convenient excuse for carelessness and neglect in the selection of the female parent. A most ingenious hypothesis baa lately been propounded by Mr. Orton of Sunderland, in a paper published in the Newcastle Chronicle of 10th March, 1854, and noticed at considerable length in the Monthly Medical Journal for August of the year. The male animal, according to Mr. Orton, influences especially the external, and the female the internal organisation of the offspring. The outward form, general appearance, and organs of locomotion are chiefly determined by the male; the vital organs, size, general vigour, and endurance by the female. Many most interesting facts, of which we subjoin few, are adduced in support of this proposition. There are many reasons for believing that Mr. Orton’s views afford a clue to an important law of physiology. But this, it must be remembered, cannot be the only law operating in the process of generation, and as Mr. Orton himself states, it must consequently be liable to many modifications, and must only be accepted with certain restrictions. Thus the parent which the time of copulation is more powerful and vigorous doubtless imparts to the progeny an unduly large share of its own prominent characters.

Mr. Harvey, in commenting on Mr. Orton’s paper, makes the following observations:— ‘The mule is the produce of the male ass and the mare; the hinny (or, as it is called, the muto), that of the horse and the she ass. Both hybrids are the produce of the same set of animals. They differ widely, however, in their respective characters—the mule, in all that relates to its external characters, having the distinctive features of the ass —the hinny, in the same respects, having all the distinctive features of the horse; while, in all that relates to the internal organs and vital qualities, the mule partakes of the characters of the horse, and the hinny of those of the ass.”

‘Mr. Orton, speaking of this, says:— “The mule, the produce of the male ass and mare, is essentially a modified ass; the ears are those of an ass, somewhat shortened; the mane is that of the ass erect; the tail is that of an ass; the skin and colour are those of an ass, somewhat modified ; the legs are slender, and the hoofs high, narrow, and contracted, like those of ass; in fact, in all these respects it is an ass somewhat modified. The body and barrel of the mule are round and full, in which it differs from the ass, and resembles the mare. The hinny (or muto), on the other hand, the produce of the stallion and she ass, is essentially modified horse; the ears are those of horse somewhat lengthened; the mane flowing; the tail is bushy, like that of the horse; the skin is finer, like that of the horse; and the colour varies also like the horse; the legs are stronger, and the hoofs broad and expanded, like those of the horse. In fact, in all these respects it is a horse somewhat modified. The body and barrel, however, of the hinny are flat and narrow, in which it differs from the horse, and resembles its mother the ass. The mule and hinny,” adds Mr. Orton, “have been selected and placed first because they afford the most conclusive evidence, and are the most familiar.”

‘Equally conclusive, although perhaps less striking, instances may be drawn from other sources. Thus it has been observed that when the Ancona, or other sheep, are allowed to breed with common ewes, the cross is not a medium between the two breeds, but that the offspring retains in great measure the short and twisted legs of the sire. Buffon made a cross between the male goat and ewe; the resulting hybrid in all the instances, which were many, were strongly characteristic of the male parent, more particularly so in the hair and length of leg. Curiously enough, the number of teats in some of the cases corresponded with those of the goat.

‘A cross between a male wolf and a bitch illustrates the same law; the offspring having a markedly wolfish aspect, skin, colour, ears, and tail. On the other hand, a cross between the dog and female wolf afforded animals much more dog-like in aspect —slouched ears and even pied in colour. If you look to the descriptions and illustrations of these two hybrids, you will perceive at a glance that the doubt arises to the mind in the case of the first, ‘What genus of this?' whereas in the case of the second, ‘What a curious mongrel dog!’

‘Amongst birds we have the same results, and they afford the like illustrations to our subject. Those who have had much to do with pigeons much have perceived that a cross between a carrier cock and a dragoon hen is always a fine bird, and very nearly equal to the carrier; whereas a cross between a dragoon cock and carrier hen results in nothing better than dragoon. Precisely the same may be observed in the cross between the tumbler and pouter.

‘lt is curious to observe,’ continues Mr. Orton, ‘that the proposition I make regarding male influence should not only have been observed, but distinctly stated in so many words. Mr. Lloyd says:—‘The capercailli occasionally breed with the black grouse, and the produce are in Sweden called racklelianen. These partake of the leading characters of both parents, but their size and colour greatly depend upon whether they have been produced between the capercailli cock and gray hen, or vice versa (Yarrell, p. 298). The hybrid between the pheasant and grouse is a striking illustration, showing so clearly its male parent: in almost all respects it is a pheasant, only the tail slightly shortened. It may be observed, too, that the feathered feet of the grouse have disappeared in the offspring (ibid. p. 309). Another instance of the same cross is given (p. 311), in which the general characteristics are those of the pheasant ; and this would have been still more striking if the tail had not been spread, a liberty, I suspect, either of the artist or the stuffer of the specimen. The legs in this instance are slightly feathered. Another hybrid is given (p. 313) between the ptarmigan and the grouse. Although the precise parentage of the bird is not stated, I am perfectly satisfied that in this case the grouse has been the male parent, and the tail indicated this, being somewhat forked and divergent. In your museum there is an interesting specimen illustrating the same law—a hybrid between the pheasant and the gray hen. In this case the produce is pheasant-like in aspect, tail like the pheasant, but somewhat spread, no appearance of forking of the tail.’

‘Even in the breeding of fish the same law has been observed. Sir Anthony Carlisle produced mule fish, by impregnating the spawn of the salmon by means of the male trout. The results I give in his own words:— ‘ These mules partook of the character of the trout more than of the salmon. They had bright red spots on their sides, but the black colour was shaded downwards in bars like those of the perch. The tails were not forked like those of the salmon, as I have seen them in the Thames skeggers (from which I infer the male salmon in that case to have been the impregnators).’ We thus see in the case of fish, as that of animals, the male parent giving the external characteristics: those produced by the male trout had not forked tails; the skeggers, on the other hand, produced by the male salmon, had forked tails.” — Finlay Dun, in Journal of the Royal Agricultural Society of England.

EXCERPTS FROM CHARLES DARWIN'S "ORIGIN OF SPECIES" (1859)

From Chapter 5: Laws of Variation

Now let us turn to the effects of crossing the several species of the horse-genus. Rollin asserts, that the common mule from the ass and horse is particularly apt to have bars on its legs. I once saw a mule with its legs so much striped that any one at first would have thought that it must have been the product of a zebra; and Mr. W. C. Martin, in his excellent treatise on the horse, has given a figure of a similar mule. In four coloured drawings, which I have seen, of hybrids between the ass and zebra, the legs were much more plainly barred than the rest of the body; and in one of them there was a double shoulder-stripe. In Lord Moreton's famous hybrid from a chestnut mare and male quagga, the hybrid, and even the pure offspring subsequently produced from the mare by a black Arabian sire, were much more plainly barred across the legs than is even the pure quagga. Lastly, and this is another most remarkable case, a hybrid has been figured by Dr. Gray (and he informs me that he knows of a second case) from the ass and the hemionus; and this hybrid, though the ass seldom has stripes on its legs and the hemionus has none and has not even a shoulder-stripe, nevertheless had all four legs barred, and had three short shoulder-stripes, like those on the dun Welch pony, and even had some zebra-like stripes on the sides of its face. With respect to this last fact, I was so convinced that not even a stripe of colour appears from what would commonly be called an accident, that I was led solely from the occurrence of the face-stripes on this hybrid from the ass and hemionus, to ask Colonel Poole whether such face-stripes ever occur in the eminently striped Kattywar breed of horses, and was, as we have seen, answered in the affirmative.

From Chapter 8: Hybridism.

The view generally entertained by naturalists is that species, when intercrossed, have been specially endowed with the quality of sterility, in order to prevent the confusion of all organic forms. This view certainly seems at first probable, for species within the same country could hardly have kept distinct had they been capable of crossing freely. The importance of the fact that hybrids are very generally sterile, has, I think, been much underrated by some late writers. On the theory of natural selection the case is especially important, inasmuch as the sterility of hybrids could not possibly be of any advantage to them, and therefore could not have been acquired by the continued preservation of successive profitable degrees of sterility. I hope, however, to be able to show that sterility is not a specially acquired or endowed quality, but is incidental on other acquired differences.

In treating this subject, two classes of facts, to a large extent fundamentally different, have generally been confounded together; namely, the sterility of two species when first crossed, and the sterility of the hybrids produced from them.

Pure species have of course their organs of reproduction in a perfect condition, yet when intercrossed they produce either few or no offspring. Hybrids, on the other hand, have their reproductive organs functionally impotent, as may be clearly seen in the state of the male element in both plants and animals; though the organs themselves are perfect in structure, as far as the microscope reveals. In the first case the two sexual elements which go to form the embryo are perfect; in the second case they are either not at all developed, or are imperfectly developed. This distinction is important, when the cause of the sterility, which is common to the two cases, has to be considered. The distinction has probably been slurred over, owing to the sterility in both cases being looked on as a special endowment, beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to have descended from common parents, when intercrossed, and likewise the fertility of their mongrel offspring, is, on my theory, of equal importance with the sterility of species; for it seems to make a broad and clear distinction between varieties and species.

[Darwin describes the work of botanists, Kolreuter and Gartner, who almost devoted their lives to the subject of plant hybrids. Both found most hybrids to be sterile, but Gartner disputed those Kolreuter found to be fertile. Reverend W. Herbert and horticulturalist Mr C Noble also found some plant hybrids to be perfectly fertile.]

In regard to animals, much fewer experiments have been carefully tried than with plants. If our systematic arrangements can be trusted, that is if the genera of animals are as distinct from each other, as are the genera of plants, then we may infer that animals more widely separated in the scale of nature can be more easily crossed than in the case of plants; but the hybrids themselves are, I think, more sterile. I doubt whether any case of a perfectly fertile hybrid animal can be considered as thoroughly well authenticated. It should, however, be borne in mind that, owing to few animals breeding freely under confinement, few experiments have been fairly tried: for instance, the canary-bird has been crossed with nine other finches, but as not one of these nine species breeds freely in confinement, we have no right to expect that the first crosses between them and the canary, or that their hybrids, should be perfectly fertile. Again, with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding [inbreeding]. On the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. If we were to act thus, and pair brothers and sisters in the case of any pure animal, which from any cause had the least tendency to sterility, the breed would assuredly be lost in a very few generations.

Although I do not know of any thoroughly well-authenticated cases of perfectly fertile hybrid animals, I have some reason to believe that the hybrids from Cervulus vaginalis [a muntjac] and Reevesii [muntjac] [...] are perfectly fertile. [Examples of fertile hybrid pheasants and geese are also given]

A doctrine which originated with Pallas, has been largely accepted by modern naturalists; namely, that most of our domestic animals have descended from two or more aboriginal species, since commingled by intercrossing. On this view, the aboriginal species must either at first have produced quite fertile hybrids, or the hybrids must have become in subsequent generations quite fertile under domestication. This latter alternative seems to me the most probable, and I am inclined to believe in its truth, although it rests on no direct evidence. I believe, for instance, that our dogs have descended from several wild stocks; yet, with perhaps the exception of certain indigenous domestic dogs of South America, all are quite fertile together; and analogy makes me greatly doubt, whether the several aboriginal species would at first have freely bred together and have produced quite fertile hybrids. So again there is reason to believe that our European and the humped Indian cattle are quite fertile together; but from facts communicated to me by Mr. Blyth, I think they must be considered as distinct species. On this view of the origin of many of our domestic animals, we must either give up the belief of the almost universal sterility of distinct species of animals when crossed; or we must look at sterility, not as an indelible characteristic, but as one capable of being removed by domestication.

Finally, looking to all the ascertained facts on the intercrossing of plants and animals, it may be concluded that some degree of sterility, both in first crosses and in hybrids, is an extremely general result; but that it cannot, under our present state of knowledge, be considered as absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We will now consider a little more in detail the circumstances and rules governing the sterility of first crosses and of hybrids. Our chief object will be to see whether or not the rules indicate that species have specially been endowed with this quality, in order to prevent their crossing and blending together in utter confusion. The following rules and conclusions are chiefly drawn up from Gartner's admirable work on the hybridisation of plants. I have taken much pains to ascertain how far the rules apply to animals, and considering how scanty our knowledge is in regard to hybrid animals, I have been surprised to find how generally the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of first crosses and of hybrids, graduates from zero to perfect fertility [the example given relates to plants].

Hybrids from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced - two classes of facts which are generally confounded together - is by no means strict. There are many cases, in which two pure species can be united with unusual facility, and produce numerous hybrid-offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dianthus [a plant], these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily affected by unfavourable conditions, than is the fertility of pure species. But the degree of fertility is likewise innately variable; for it is not always the same when the same two species are crossed under the same circumstances, but depends in part upon the constitution of the individuals which happen to have been chosen for the experiment. So it is with hybrids, for their degree of fertility is often found to differ greatly in the several individuals raised from seed out of the same capsule and exposed to exactly the same conditions.

By the term systematic affinity is meant, the resemblance between species in structure and in constitution, more especially in the structure of parts which are of high physiological importance and which differ little in the allied species. Now the fertility of first crosses between species, and of the hybrids produced from them, is largely governed by their systematic affinity. [The example given related to plants]

No one has been able to point out what kind, or what amount, of difference in any recognisable character is sufficient to prevent two species crossing. [This related to plants]

By a reciprocal cross between two species, I mean the case, for instance, of a stallion-horse being first crossed with a female-ass, and then a male-ass with a mare: these two species may then be said to have been reciprocally crossed. There is often the widest possible difference in the facility of making reciprocal crosses. Such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity, or of any recognisable difference in their whole organisation. On the other hand, these cases clearly show that the capacity for crossing is connected with constitutional differences imperceptible by us, and confined to the reproductive system. [...] It is also a remarkable fact, that hybrids raised from reciprocal crosses, though of course compounded of the very same two species, the one species having first been used as the father and then as the mother, generally differ in fertility in a small, and occasionally in a high degree.

Several other singular rules could be given from Gartner: for instance, some species have a remarkable power of crossing with other species; other species of the same genus have a remarkable power of impressing their likeness on their hybrid offspring; but these two powers do not at all necessarily go together. There are certain hybrids which instead of having, as is usual, an intermediate character between their two parents, always closely resemble one of them; and such hybrids, though externally so like one of their pure parent-species, are with rare exceptions extremely sterile. So again amongst hybrids which are usually intermediate in structure between their parents, exceptional and abnormal individuals sometimes are born, which closely resemble one of their pure parents; and these hybrids are almost always utterly sterile, even when the other hybrids raised from seed from the same capsule have a considerable degree of fertility. These facts show how completely fertility in the hybrid is independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fertility of first crosses and of hybrids, we see that when forms, which must be considered as good and distinct species, are united, their fertility graduates from zero to perfect fertility, or even to fertility under certain conditions in excess. That their fertility, besides being eminently susceptible to favourable and unfavourable conditions, is innately variable. That it is by no means always the same in degree in the first cross and in the hybrids produced from this cross. That the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent. And lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other. This latter statement is clearly proved by reciprocal crosses between the same two species, for according as the one species or the other is used as the father or the mother, there is generally some difference, and occasionally the widest possible difference, in the facility of effecting an union. The hybrids, moreover, produced from reciprocal crosses often differ in fertility.

Now do these complex and singular rules indicate that species have been endowed with sterility simply to prevent their becoming confounded in nature? I think not. For why should the sterility be so extremely different in degree, when various species are crossed, all of which we must suppose it would be equally important to keep from blending together? Why should the degree of sterility be innately variable in the individuals of the same species? Why should some species cross with facility, and yet produce very sterile hybrids; and other species cross with extreme difficulty, and yet produce fairly fertile hybrids? Why should there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems to be a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me clearly to indicate that the sterility both of first crosses and of hybrids is simply incidental or dependent on unknown differences, chiefly in the reproductive systems, of the species which are crossed. The differences being of so peculiar and limited a nature, that, in reciprocal crosses between two species the male sexual element of the one will often freely act on the female sexual element of the other, but not in a reversed direction. [A plant example was given and included grafting as a means of determining compatibility.]

CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS.

We may now look a little closer at the probable causes of the sterility of first crosses and of hybrids. These two cases are fundamentally different, for, as just remarked, in the union of two pure species the male and female sexual elements are perfect, whereas in hybrids they are imperfect. Even in first crosses, the greater or lesser difficulty in effecting a union apparently depends on several distinct causes. [Plant examples are given: physical impossibility in the male penetrating the female, the male gamete being unable to penetrate the ovum, failure to fertilise the ovum, death of the embryo at an early stage] I was at first very unwilling to believe in this view [ the early death of the embryo]; as hybrids, when once born, are generally healthy and long-lived, as we see in the case of the common mule. Hybrids, however, are differently circumstanced before and after birth: when born and living in a country where their two parents can live, they are generally placed under suitable conditions of life. But a hybrid partakes of only half of the nature and constitution of its mother, and therefore before birth, as long as it is nourished within its mother's womb or within the egg or seed produced by the mother, it may be exposed to conditions in some degree unsuitable, and consequently be liable to perish at an early period; more especially as all very young beings seem eminently sensitive to injurious or unnatural conditions of life.

In regard to the sterility of hybrids, in which the sexual elements are imperfectly developed, the case is very different. I have more than once alluded to a large body of facts, which I have collected, showing that when animals and plants are removed from their natural conditions, they are extremely liable to have their reproductive systems seriously affected. This, in fact, is the great bar to the domestication of animals. Between the sterility thus superinduced and that of hybrids, there are many points of similarity. In both cases the sterility is independent of general health, and is often accompanied by excess of size or great luxuriance. In both cases, the sterility occurs in various degrees; in both, the male element is the most liable to be affected; but sometimes the female more than the male. In both, the tendency goes to a certain extent with systematic affinity, for whole groups of animals and plants are rendered impotent by the same unnatural conditions; and whole groups of species tend to produce sterile hybrids. On the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility; and certain species in a group will produce unusually fertile hybrids. No one can tell, till he tries, whether any particular animal will breed under confinement or any plant seed freely under culture; nor can he tell, till he tries, whether any two species of a genus will produce more or less sterile hybrids. Lastly, when organic beings are placed during several generations under conditions not natural to them, they are extremely liable to vary, which is due, as I believe, to their reproductive systems having been specially affected, though in a lesser degree than when sterility ensues. So it is with hybrids, for hybrids in successive generations are eminently liable to vary, as every experimentalist has observed.

Thus we see that when organic beings are placed under new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, independently of the general state of health, is affected by sterility in a very similar manner. In the one case, the conditions of life have been disturbed, though often in so slight a degree as to be inappreciable by us; in the other case, or that of hybrids, the external conditions have remained the same, but the organisation has been disturbed by two different structures and constitutions having been blended into one. For it is scarcely possible that two organisations should be compounded into one, without some disturbance occurring in the development, or periodical action, or mutual relation of the different parts and organs one to another, or to the conditions of life. When hybrids are able to breed inter se [among themselves], they transmit to their offspring from generation to generation the same compounded organisation, and hence we need not be surprised that their sterility, though in some degree variable, rarely diminishes.

It must, however, be confessed that we cannot understand, excepting on vague hypotheses, several facts with respect to the sterility of hybrids; for instance, the unequal fertility of hybrids produced from reciprocal crosses; or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. Nor do I pretend that the foregoing remarks go to the root of the matter: no explanation is offered why an organism, when placed under unnatural conditions, is rendered sterile. All that I have attempted to show, is that in two cases, in some respects allied, sterility is the common result, - in the one case from the conditions of life having been disturbed, in the other case from the organisation having been disturbed by two organisations having been compounded into one.

It may seem fanciful, but I suspect that a similar parallelism extends to an allied yet very different class of facts. It is an old and almost universal belief, founded, I think, on a considerable body of evidence, that slight changes in the conditions of life are beneficial to all living things. We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers, etc., from one soil or climate to another, and back again. During the convalescence of animals, we plainly see that great benefit is derived from almost any change in the habits of life. Again, both with plants and animals, there is abundant evidence, that a cross between very distinct individuals of the same species, that is between members of different strains or sub-breeds, gives vigour and fertility to the offspring. I believe, indeed, from the facts alluded to in our fourth chapter, that a certain amount of crossing is indispensable even with hermaphrodites; and that close interbreeding continued during several generations between the nearest relations, especially if these be kept under the same conditions of life, always induces weakness and sterility in the progeny.

Hence it seems that, on the one hand, slight changes in the conditions of life benefit all organic beings, and on the other hand, that slight crosses, that is crosses between the males and females of the same species which have varied and become slightly different, give vigour and fertility to the offspring. But we have seen that greater changes, or changes of a particular nature, often render organic beings in some degree sterile; and that greater crosses, that is crosses between males and females which have become widely or specifically different, produce hybrids which are generally sterile in some degree. I cannot persuade myself that this parallelism is an accident or an illusion. Both series of facts seem to be connected together by some common but unknown bond, which is essentially related to the principle of life.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL OFFSPRING.

It may be urged, as a most forcible argument, that there must be some essential distinction between species and varieties, and that there must be some error in all the foregoing remarks, inasmuch as varieties, however much they may differ from each other in external appearance, cross with perfect facility, and yield perfectly fertile offspring. I fully admit that this is almost invariably the case. But if we look to varieties produced under nature, we are immediately involved in hopeless difficulties; for if two hitherto reputed varieties be found in any degree sterile together, they are at once ranked by most naturalists as species.

If we turn to varieties, produced, or supposed to have been produced, under domestication, we are still involved in doubt. For when it is stated, for instance, that the German Spitz dog unites more easily than other dogs with foxes, or that certain South American indigenous domestic dogs do not readily cross with European dogs, the explanation which will occur to everyone, and probably the true one, is that these dogs have descended from several aboriginally distinct species. Nevertheless the perfect fertility of so many domestic varieties, differing widely from each other in appearance, for instance of the pigeon or of the cabbage, is a remarkable fact; more especially when we reflect how many species there are, which, though resembling each other most closely, are utterly sterile when intercrossed. Several considerations, however, render the fertility of domestic varieties less remarkable than at first appears. It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile; and if this be so, we surely ought not to expect to find sterility both appearing and disappearing under nearly the same conditions of life. Lastly, and this seems to me by far the most important consideration, new races of animals and plants are produced under domestication by man's methodical and unconscious power of selection, for his own use and pleasure: he neither wishes to select, nor could select, slight differences in the reproductive system, or other constitutional differences correlated with the reproductive system. He supplies his several varieties with the same food; treats them in nearly the same manner, and does not wish to alter their general habits of life. Nature acts uniformly and slowly during vast periods of time on the whole organisation, in any way which may be for each creature's own good; and thus she may, either directly, or more probably indirectly, through correlation, modify the reproductive system in the several descendants from any one species. Seeing this difference in the process of selection, as carried on by man and nature, we need not be surprised at some difference in the result.

I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed. But it seems to me impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases, which I will briefly abstract. The evidence is at least as good as that from which we believe in the sterility of a multitude of species. The evidence is, also, derived from hostile witnesses, who in all other cases consider fertility and sterility as safe criterions of specific distinction. [Plant examples are given where supposed species are interfertile and their hybrids are fertile]

From these facts; from the great difficulty of ascertaining the infertility of varieties in a state of nature, for a supposed variety if infertile in any degree would generally be ranked as species; from man selecting only external characters in the production of the most distinct domestic varieties, and from not wishing or being able to produce recondite and functional differences in the reproductive system; from these several considerations and facts, I do not think that the very general fertility of varieties can be proved to be of universal occurrence, or to form a fundamental distinction between varieties and species. The general fertility of varieties does not seem to me sufficient to overthrow the view which I have taken with respect to the very general, but not invariable, sterility of first crosses and of hybrids, namely, that it is not a special endowment, but is incidental on slowly acquired modifications, more especially in the reproductive systems of the forms which are crossed.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF THEIR FERTILITY.

Independently of the question of fertility, the offspring of species when crossed and of varieties when crossed may be compared in several other respects. Gartner, whose strong wish was to draw a marked line of distinction between species and varieties, could find very few and, as it seems to me, quite unimportant differences between the so-called hybrid offspring of species, and the so-called mongrel offspring of varieties. And, on the other hand, they agree most closely in very many important respects.

I shall here discuss this subject with extreme brevity. The most important distinction is, that in the first generation mongrels are more variable than hybrids; but Gartner admits that hybrids from species which have long been cultivated are often variable in the first generation; and I have myself seen striking instances of this fact. Gartner further admits that hybrids between very closely allied species are more variable than those from very distinct species; and this shows that the difference in the degree of variability graduates away. When mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in their offspring is notorious; but some few cases both of hybrids and mongrels long retaining uniformity of character could be given. The variability, however, in the successive generations of mongrels is, perhaps, greater than in hybrids.

This greater variability of mongrels than of hybrids does not seem to me at all surprising. For the parents of mongrels are varieties, and mostly domestic varieties (very few experiments having been tried on natural varieties), and this implies in most cases that there has been recent variability; and therefore we might expect that such variability would often continue and be super-added to that arising from the mere act of crossing. The slight degree of variability in hybrids from the first cross or in the first generation, in contrast with their extreme variability in the succeeding generations, is a curious fact and deserves attention. For it bears on and corroborates the view which I have taken on the cause of ordinary variability; namely, that it is due to the reproductive system being eminently sensitive to any change in the conditions of life, being thus often rendered either impotent or at least incapable of its proper function of producing offspring identical with the parent-form. Now hybrids in the first generation are descended from species (excluding those long cultivated) which have not had their reproductive systems in any way affected, and they are not variable; but hybrids themselves have their reproductive systems seriously affected, and their descendants are highly variable.

But to return to our comparison of mongrels and hybrids: Gartner states that mongrels are more liable than hybrids to revert to either parent-form; but this, if it be true, is certainly only a difference in degree. Gartner further insists that when any two species, although most closely allied to each other, are crossed with a third species, the hybrids are widely different from each other; whereas if two very distinct varieties of one species are crossed with another species, the hybrids do not differ much. But this conclusion, as far as I can make out, is founded on a single experiment; and seems directly opposed to the results of several experiments made by Kolreuter.

These alone are the unimportant differences, which Gartner is able to point out, between hybrid and mongrel plants. On the other hand, the resemblance in mongrels and in hybrids to their respective parents, more especially in hybrids produced from nearly related species, follows according to Gartner the same laws. When two species are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid; and so I believe it to be with varieties of plants. With animals one variety certainly often has this prepotent power over another variety. Hybrid plants produced from a reciprocal cross, generally resemble each other closely; and so it is with mongrels from a reciprocal cross. Both hybrids and mongrels can be reduced to either pure parent-form, by repeated crosses in successive generations with either parent.

These several remarks are apparently applicable to animals; but the subject is here excessively complicated, partly owing to the existence of secondary sexual characters; but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other, both when one species is crossed with another, and when one variety is crossed with another variety. For instance, I think those authors are right, who maintain that the ass has a prepotent power over the horse, so that both the mule and the hinny more resemble the ass than the horse; but that the prepotency runs more strongly in the male-ass than in the female, so that the mule, which is the offspring of the male-ass and mare, is more like an ass, than is the hinny, which is the offspring of the female-ass and stallion.

Much stress has been laid by some authors on the supposed fact, that mongrel animals alone are born closely like one of their parents; but it can be shown that this does sometimes occur with hybrids; yet I grant much less frequently with hybrids than with mongrels. Looking to the cases which I have collected of cross-bred animals closely resembling one parent, the resemblances seem chiefly confined to characters almost monstrous in their nature, and which have suddenly appeared - such as albinism, melanism, deficiency of tail or horns, or additional fingers and toes; and do not relate to characters which have been slowly acquired by selection. Consequently, sudden reversions to the perfect character of either parent would be more likely to occur with mongrels, which are descended from varieties often suddenly produced and semi-monstrous in character, than with hybrids, which are descended from species slowly and naturally produced. On the whole I entirely agree with Dr. Prosper Lucas, who, after arranging an enormous body of facts with respect to animals, comes to the conclusion, that the laws of resemblance of the child to its parents are the same, whether the two parents differ much or little from each other, namely in the union of individuals of the same variety, or of different varieties, or of distinct species.

Laying aside the question of fertility and sterility, in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. If we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species and varieties.

SUMMARY OF CHAPTER.

First crosses between forms sufficiently distinct to be ranked as species, and their hybrids, are very generally, but not universally, sterile. The sterility is of all degrees, and is often so slight that the two most careful experimentalists who have ever lived, have come to diametrically opposite conclusions in ranking forms by this test. The sterility is innately variable in individuals of the same species, and is eminently susceptible of favourable and unfavourable conditions. The degree of sterility does not strictly follow systematic affinity, but is governed by several curious and complex laws. It is generally different, and sometimes widely different, in reciprocal crosses between the same two species. It is not always equal in degree in a first cross and in the hybrid produced from this cross.

The sterility of first crosses between pure species, which have their reproductive systems perfect, seems to depend on several circumstances; in some cases largely on the early death of the embryo. The sterility of hybrids, which have their reproductive systems imperfect, and which have had this system and their whole organisation disturbed by being compounded of two distinct species, seems closely allied to that sterility which so frequently affects pure species, when their natural conditions of life have been disturbed. This view is supported by a parallelism of another kind; - namely, that the crossing of forms only slightly different is favourable to the vigour and fertility of their offspring; and that slight changes in the conditions of life are apparently favourable to the vigour and fertility of all organic beings. It is not surprising that the degree of difficulty in uniting two species, and the degree of sterility of their hybrid-offspring should generally correspond, though due to distinct causes; for both depend on the amount of difference of some kind between the species which are crossed. Nor is it surprising that the facility of effecting a first cross, the fertility of the hybrids produced, [and ability to be grafted together in plants] should all run, to a certain extent, parallel with the systematic affinity of the forms which are subjected to experiment; for systematic affinity attempts to express all kinds of resemblance between all species.

First crosses between forms known to be varieties, or sufficiently alike to be considered as varieties, and their mongrel offspring, are very generally, but not quite universally, fertile. Nor is this nearly general and perfect fertility surprising, when we remember how liable we are to argue in a circle with respect to varieties in a state of nature; and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences, and not of differences in the reproductive system. In all other respects, excluding fertility, there is a close general resemblance between hybrids and mongrels. Finally, then, the facts briefly given in this chapter do not seem to me opposed to, but even rather to support the view, that there is no fundamental distinction between species and varieties.

EXCERPTS FROM "DARWINISM AN EXPOSITION OF THE THEORY OF NATURAL SELECTION WITH SOME OF ITS APPLICATIONS"
BY ALFRED RUSSEL WALLACE (1889)

One of the greatest, or perhaps we may say the greatest, of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed. Generally speaking, it may be said that the varieties of any one species, however different they may be in external appearance, are perfectly fertile when crossed, and their mongrel offspring are equally fertile when bred among themselves; while distinct species, on the other hand, however closely they may resemble each other externally, are usually infertile when crossed, and their hybrid offspring absolutely sterile. This used to be considered a fixed law of nature, constituting the absolute test and criterion of a species as distinct from a variety; and so long as it was believed that species were separate creations, or at all events had an origin quite distinct from that of varieties, this law could have no exceptions, because, if any two species had been found to be fertile when crossed and their hybrid offspring to be also fertile, this fact would have been held to prove them to be not species but varieties. On the other hand, if two varieties had been found to be infertile, or their mongrel offspring to be sterile, then it would have been said: These are not varieties but true species. Thus the old theory led to inevitable reasoning in a circle; and what might be only a rather common fact was elevated into a law which had no exceptions.

The elaborate and careful examination of the whole subject by Mr. Darwin, who has brought together a vast mass of evidence from the experience of agriculturists and horticulturists, as well as from scientific experimenters, has demonstrated that there is no such fixed law in nature as was formerly supposed. He shows us that crosses between some varieties are infertile or even sterile, while crosses between some species are quite fertile; and that there are besides a number of curious phenomena connected with the subject which render it impossible to believe that sterility is anything more than an incidental property of species, due to the extreme delicacy and susceptibility of the reproductive powers, and dependent on physiological causes we have not yet been able to trace. Nevertheless, the fact remains that most species which have hitherto been crossed produce sterile hybrids, as in the well-known case of the mule; while almost all domestic varieties, when crossed, produce offspring which are perfectly fertile among themselves. I will now endeavour to give such a sketch of the subject as may enable the reader to see something of the complexity of the problem, referring him to Mr. Darwin's works for fuller details.

EXTREME SUSCEPTIBILITY OF THE REPRODUCTIVE FUNCTIONS.

One of the most interesting facts, as showing how susceptible to changed conditions or to slight constitutional changes are the reproductive powers of animals, is the very general difficulty of getting those which are kept in confinement to breed; and this is frequently the only bar to domesticating wild species. Thus, elephants, bears, foxes, and numbers of species of rodents, very rarely breed in confinement; while other species do so more or less freely. [...] This inability to reproduce is not due to ill-health, since many of these creatures are perfectly vigorous and live very long.

With our true domestic animals, on the other hand, fertility is perfect, and is very little affected by changed conditions. [...] It therefore seems probable, that this facility for breeding under changed conditions was an original property of the species which man has domesticated - a property which, more than any other, enabled him to domesticate them. Yet, even with these, there is evidence that great changes of conditions affect the fertility. In the hot valleys of the Andes sheep are less fertile; while geese taken to the high plateau of Bogota were at first almost sterile, but after some generations recovered their fertility. These and many other facts seem to show that, with the majority of animals, even a slight change of conditions may produce infertility or sterility; and also that after a time, when the animal has become thoroughly acclimatised, as it were, to the new conditions, the infertility is in some cases diminished or altogether ceases. It is stated by Bechstein that the canary was long infertile, and it is only of late years that good breeding birds have become common; but in this case no doubt selection has aided the change.

RECIPROCAL CROSSES.

Another indication of the extreme delicacy of the adjustment between the sexes, which is necessary to produce fertility, is afforded by the behaviour of many species and varieties when reciprocally crossed.

INDIVIDUAL DIFFERENCES IN RESPECT TO CROSS-FERTILISATION.

A still more remarkable illustration of the delicate balance of organisation needful for reproduction, is afforded by the individual differences of animals and plants, as regards both their power of intercrossing with other individuals or other species, and the fertility of the offspring thus produced. Among domestic animals, Darwin states that it is by no means rare to find certain males and females which will not breed together, though both are known to be perfectly fertile with other males and females. Cases of this kind have occurred among horses, cattle, pigs, dogs, and pigeons; and the experiment has been tried so frequently that there can be no doubt of the fact. Professor G.J. Romanes states that he has a number of additional cases of this individual incompatibility, or of absolute sterility, between two individuals, each of which is perfectly fertile with other individuals.

CASES OF THE FERTILITY OF HYBRIDS, AND OF THE INFERTILITY OF MONGRELS.

I now propose to adduce a few cases in which it has been proved, by experiment, that hybrids between two distinct species are fertile inter se; and then to consider why it is that such cases are so few in number. [Case 1 is the common domestic goose (Anser ferns) and the Chinese goose (A. cygnoides)]

Another equally striking case is that of the Indian humped and the common cattle, species which differ osteologically, and also in habits, form, voice, and constitution, so that they are by no means closely allied; yet Mr. Darwin assures us that he has received decisive evidence that the hybrids between these are perfectly fertile inter se.

Dogs have been frequently crossed with wolves and with jackals, and their hybrid offspring have been found to be fertile inter se [among themselves] to the third or fourth generation, and then usually to show some signs of sterility or of deterioration. The wolf and dog may be originally the same species, but the jackal is certainly distinct; and the appearance of infertility or of weakness is probably due to the fact that, in almost all these experiments, the offspring of a single pair - themselves usually from the same litter - - were bred in-and-in, and this alone sometimes produces the most deleterious effects. Thus, Mr. Low in his great work on the Domesticated Animals of Great Britain, says: "If we shall breed a pair of dogs from the same litter, and unite again the offspring of this pair, we shall produce at once a feeble race of creatures; and the process being repeated for one or two generations more, the family will die out, or be incapable of propagating their race. A gentleman of Scotland made the experiment on a large scale with certain foxhounds, and he found that the race actually became monstrous and perished utterly." The same writer tells us that hogs have been made the subject of similar experiments: "After a few generations the victims manifest the change induced in the system. They become of diminished size; the bristles are changed into hairs; the limbs become feeble and short; the litters diminish in frequency, and in the number of the young produced; the mother becomes unable to nourish them, and, if the experiment be carried as far as the case will allow, the feeble, and frequently monstrous offspring, will be incapable of being reared up, and the miserable race will utterly perish."

These precise statements, by one of the greatest authorities on our domesticated animals, are sufficient to show that the fact of infertility or degeneracy appearing in the offspring of hybrids after a few generations need not be imputed to the fact of the first parents being distinct species, since exactly the same phenomena appear when individuals of the same species are bred under similar adverse conditions. But in almost all the experiments that have hitherto been made in crossing distinct species, no care has been taken to avoid close interbreeding by securing several hybrids from quite distinct stocks to start with, and by having two or more sets of experiments carried on at once, so that crosses between the hybrids produced may be occasionally made. Till this is done no experiments, such as those hitherto tried, can be held to prove that hybrids are in all cases infertile inter se [among themselves].

It has, however, been denied by Mr. A.H. Huth, in his interesting work on The Marriage of Near Kin, that any amount of breeding in-and-in is in itself hurtful; and he quotes the evidence of numerous breeders whose choicest stocks have always been so bred, as well as cases like the Porto Santo rabbits, the goats of Juan Fernandez, and other cases in which animals allowed to run wild have increased prodigiously and continued in perfect health and vigour, although all derived from a single pair. But in all these cases there has been rigid selection by which the weak or the infertile have been eliminated, and with such selection there is no doubt that the ill effects of close interbreeding can be prevented for a long time; but this by no means proves that no ill effects are produced. Mr. Huth himself quotes M. Allié, M. Aubé, Stephens, Giblett, Sir John Sebright, Youatt, Druce, Lord Weston, and other eminent breeders, as finding from experience that close interbreeding does produce bad effects; and it cannot be supposed that there would be such a consensus of opinion on this point if the evil were altogether imaginary. Mr. Huth argues, that the evil results which do occur do not depend on the close interbreeding itself, but on the tendency it has to perpetuate any constitutional weakness or other hereditary taints; and he attempts to prove this by the argument that "if crosses act by virtue of being a cross, and not by virtue of removing an hereditary taint, then the greater the difference between the two animals crossed the more beneficial will that act be." He then shows that, the wider the difference the less is the benefit, and concludes that a cross, as such, has no beneficial effect. A parallel argument would be, that change of air, as from inland to the sea-coast, or from a low to an elevated site, is not beneficial in itself, because, if so, a change to the tropics or to the polar regions should be more beneficial. In both these cases it may well be that no benefit would accrue to a person in perfect health; but then there is no such thing as "perfect health" in man, and probably no such thing as absolute freedom from constitutional taint in animals. The experiments of Mr. Darwin, showing the great and immediate good effects of a cross between distinct strains in plants, cannot be explained away; neither can the innumerable arrangements to secure cross-fertilisation by insects, the real use and purport of which will be discussed in our eleventh chapter. On the whole, then, the evidence at our command proves that, whatever may be its ultimate cause, close interbreeding does usually produce bad results; and it is only by the most rigid selection, whether natural or artificial, that the danger can be altogether obviated.

FERTILE HYBRIDS AMONG ANIMALS.

One or two more cases of fertile hybrids may be given before we pass on to the corresponding experiments in plants. Professor Alfred Newton received from a friend a pair of hybrid ducks, bred from a common duck (Anas boschas), and a pintail (Dafila acuta). From these he obtained four ducklings, but these latter, when grown up, proved infertile, and did not breed again. In this case we have the results of close interbreeding, with too great a difference between the original species, combining to produce infertility, yet the fact of a hybrid from such a pair producing healthy offspring is itself noteworthy.

Still more extraordinary is the following statement of Mr. Low: "It has been long known to shepherds, though questioned by naturalists, that the progeny of the cross between the sheep and goat is fertile. Breeds of this mixed race are numerous in the north of Europe." Nothing appears to be known of such hybrids either in Scandinavia or in Italy; but Professor Giglioli of Florence has kindly given me some useful references to works in which they are described. The following extract from his letter is very interesting: "I need not tell you that there being such hybrids is now generally accepted as a fact. Buffon (Supplements, tom. iii. p. 7, 1756) obtained one such hybrid in 1751 and eight in 1752. Sanson (La Culture, vol. vi. p. 372, 1865) mentions a case observed in the Vosges, France. Geoff. St. Hilaire (Hist. Nat. Gén. des reg. org., vol. iii. p. 163) was the first to mention, I believe, that in different parts of South America the ram is more usually crossed with the she-goat than the sheep with the he-goat. The well-known 'pellones' of Chile are produced by the second and third generation of such hybrids (Gay, 'Hist, de Chile,' vol. i. p. 466, Agriculture, 1862). Hybrids bred from goat and sheep are called 'chabin' in French, and 'cabruno' in Spanish. In Chile such hybrids are called 'carneros lanudos'; their breeding inter se [among themselves] appears to be not always successful, and often the original cross has to be recommenced to obtain the proportion of three-eighths of he-goat and five-eighths of sheep, or of three-eighths of ram and five-eighths of she-goat; such being the reputed best hybrids."

With these numerous facts recorded by competent observers we can hardly doubt that races of hybrids between these very distinct species have been produced, and that such hybrids are fairly fertile inter se [among themselves]; and the analogous facts already given lead us to believe that whatever amount of infertility may at first exist could be eliminated by careful selection, if the crossed races were bred in large numbers and over a considerable area of country. This case is especially valuable, as showing how careful we should be in assuming the infertility of hybrids when experiments have been made with the progeny of a single pair, and have been continued only for one or two generations.

Among insects one case only appears to have been recorded. The hybrids of two moths (Bombyx cynthia and B. arrindia) were proved in Paris, according to M. Quatrefages, to be fertile inter se [among themselves] for eight generations.

CASES OF STERILITY OF MONGRELS.

The reverse phenomenon to the fertility of hybrids, the sterility of mongrels or of the crosses between varieties of the same species, is a comparatively rare one, yet some undoubted cases have occurred [plant examples were given].

No cases of this kind are recorded among animals; but this is not to be wondered at, when we consider how very few experiments have been made with natural varieties; while there is good reason for believing that domestic varieties are exceptionally fertile, partly because one of the conditions of domestication was fertility under changed conditions, and also because long continued domestication is believed to have the effect of increasing fertility and eliminating whatever sterility may exist. This is shown by the fact that, in many cases, domestic animals are descended from two or more distinct species. This is almost certainly the case with the dog, and probably with the hog, the ox, and the sheep; yet the various breeds are now all perfectly fertile, although we have every reason to suppose that there would be some degree of infertility if the several aboriginal species were crossed together for the first time.

REMARKS ON THE FACTS OF HYBRIDITY.

The facts that have now been adduced, though not very numerous, are sufficiently conclusive to prove that the old belief, of the universal sterility of hybrids and fertility of mongrels, is incorrect. The doctrine that such a universal law existed was never more than a plausible generalisation, founded on a few inconclusive facts derived from domesticated animals and cultivated plants. The facts were, and still are, inconclusive for several reasons. They are founded, primarily, on what occurs among animals in domestication; and it has been shown that domestication both tends to increase fertility, and was itself rendered possible by the fertility of those particular species being little affected by changed conditions. The exceptional fertility of all the varieties of domesticated animals does not prove that a similar fertility exists among natural varieties. In the next place, the generalisation is founded on too remote crosses, as in the case of the horse and the ass, the two most distinct and widely separated species of the genus Equus, so distinct indeed that they have been held by some naturalists to form distinct genera. Crosses between the two species of zebra, or even between the zebra and the quagga, or the quagga and the ass, might have led to a very different result. Again, in pre-Darwinian times it was so universally the practice to argue in a circle, and declare that the fertility of the offspring of a cross proved the identity of species of the parents, that experiments in hybridity were usually made between very remote species and even between species of different genera, to avoid the possibility of the reply: "They are both really the same species;" and the sterility of the hybrid offspring of such remote crosses of course served to strengthen the popular belief.

Now that we have arrived at a different standpoint, and look upon a species, not as a distinct entity due to special creation, but as an assemblage of individuals which have become somewhat modified in structure, form, and constitution so as to adapt them to slightly different conditions of life; which can be differentiated from other allied assemblages; which reproduce their like, and which usually breed together - we require a fresh set of experiments calculated to determine the matter of fact, - whether such species crossed with their near allies do always produce offspring which are more or less sterile inter se [among themselves]. Ample materials for such experiments exist, in the numerous "representative species" inhabiting distinct areas on a continent or different islands of a group; or even in those found in the same area but frequenting somewhat different stations.

To carry out these experiments with any satisfactory result, it will be necessary to avoid the evil effects of confinement and of too close interbreeding. If birds are experimented with, they should be allowed as much liberty as possible, a plot of ground with trees and bushes being enclosed with wire netting overhead so as to form a large open aviary. The species experimented with should be obtained in considerable numbers, and by two separate persons, each making the opposite reciprocal cross, as explained at p. 155. In the second generation these two stocks might be themselves crossed to prevent the evil effects of too close interbreeding. By such experiments, carefully carried out with different groups of animals and plants, we should obtain a body of facts of a character now sadly wanting, and without which it is hopeless to expect to arrive at a complete solution of this difficult problem. There are, however, some other aspects of the question that need to be considered, and some theoretical views which require to be carefully examined, having done which we shall be in a condition to state the general conclusions to which the facts and reasonings at our command seem to point.

STERILITY DUE TO CHANGED CONDITIONS AND USUALLY CORRELATED WITH OTHER CHARACTERS, ESPECIALLY WITH COLOUR.

The evidence already adduced as to the extreme susceptibility of the reproductive system, and the curious irregularity with which infertility or sterility appears in the crosses between some varieties or species while quite absent in those between others, seem to indicate that sterility is a characteristic which has a constant tendency to appear, either by itself or in correlation with other characters. It is known to be especially liable to occur under changed conditions of life; and, as such change is usually the starting-point and cause of the development of new species, we have already found a reason why it should so often appear when species become fully differentiated.

In almost all the cases of infertility or sterility between varieties or species, we have some external differences with which it is correlated; and though these differences are sometimes slight, and the amount of the infertility is not always, or even usually, proportionate to the external difference between the two forms crossed, we must believe that there is some connection between the two classes of facts. This is especially the case as regards colour; and Mr. Darwin has collected a body of facts which go far to prove that colour, instead of being an altogether trifling and unimportant character, as was supposed by the older naturalists, is really one of great significance, since it is undoubtedly often correlated with important constitutional differences [plant examples are given].

Similar phenomena have not been recorded among animals; but this is not to be wondered at when we consider that most of our pure and valued domestic breeds are characterised by definite colours which constitute one of their distinctive marks, and they are, therefore, seldom crossed with these of another colour; and even when they are so crossed, no notice would be taken of any slight diminution of fertility, since this is liable to occur from many causes. We have also reason to believe that fertility has been increased by long domestication, in addition to the fact of the original stocks being exceptionally fertile; and no experiments have been made on the differently coloured varieties of wild animals. There are, however, a number of very curious facts showing that colour in animals, as in plants, is often correlated with constitutional differences of a remarkable kind, and as these have a close relation to the subject we are discussing, a brief summary of them will be here given.

CORRELATION OF COLOUR WITH CONSTITUTIONAL PECULIARITIES.

The correlation of a white colour and blue eyes in male cats with deafness, and of the tortoise-shell marking with the female sex of the same animal, are two well-known but most extraordinary cases. Equally remarkable is the fact, communicated to Darwin by Mr. Tegetmeier, that white, yellow, pale blue, or dun pigeons, of all breeds, have the young birds born naked, while in all other colours they are well covered with down. Here we have a case in which colour seems of more physiological importance than all the varied structural differences between the varieties and breeds of pigeons. In Virginia there is a plant called the paint-root (Lachnanthes tinctoria), which, when eaten by pigs, colours their bones pink, and causes the hoofs of all but the black varieties to drop off; so that black pigs only can be kept in the district.[58] Buckwheat in flower is also said to be injurious to white pigs but not to black. In the Tarentino, black sheep are not injured by eating the Hypericum crispum - a species of St. John's-wort - which kills white sheep. White terriers suffer most from distemper; white chickens from the gapes. White-haired horses or cattle are subject to cutaneous diseases from which the dark coloured are free; while, both in Thuringia and the West Indies, it has been noticed that white or pale coloured cattle are much more troubled by flies than are those which are brown or black.

These curious and inexplicable correlations of colour with constitutional peculiarities, both in animals and plants, render it probable that the correlation of colour with infertility, which has been detected in several cases in plants, may also extend to animals in a state of nature; and if so, the fact is of the highest importance as throwing light on the origin of the infertility of many allied species. This will be better understood after considering the facts which will be now described.

Footnote to section: In the Medico-Chirurgical Transactions, vol. liii. (1870), Dr. Ogle has adduced some curious physiological facts bearing on the presence or absence of white colours in the higher animals. He states that a dark pigment in the olfactory region of the nostrils is essential to perfect smell, and that this pigment is rarely deficient except when the whole animal is pure white, and the creature is then almost without smell or taste. He observes that there is no proof that, in any of the cases given above, the black animals actually eat the poisonous root or plant; and that the facts are readily understood if the senses of smell and taste are dependent on a pigment which is absent in the white animals, who therefore eat what those gifted with normal senses avoid. This explanation however hardly seems to cover the facts. We cannot suppose that almost all the sheep in the world (which are mostly white) are without smell or taste. The cutaneous disease on the white patches of hair on horses, the special liability of white terriers to distemper, of white chickens to the gapes, and of silkworms which produce yellow silk to the fungus, are not explained by it. The analogous facts in plants also indicate a real constitutional relation with colour, not an affection of the sense of smell and taste only.

THE ISOLATION OF VARIETIES BY SELECTIVE ASSOCIATION.

In the last chapter I have shown that the importance of geographical isolation for the formation of new species by natural selection has been greatly exaggerated, because the very change of conditions, which is the initial power in starting such new forms, leads also to a local or stational segregation of the forms acted upon. But there is also a very powerful cause of isolation in the mental nature - the likes and dislikes - of animals; and to this is probably due the fact of the comparative rarity of hybrids in a state of nature. The differently coloured herds of cattle in the Falkland Islands, each of which keeps separate, have been already mentioned; and it may be added, that the mouse-coloured variety seem to have already developed a physiological peculiarity in breeding a month earlier than the others. Similar facts occur, however, among our domestic animals and are well known to breeders. Professor Low, one of the greatest authorities on our domesticated animals, says: "The female of the dog, when not under restraint, makes selection of her mate, the mastiff selecting the mastiff, the terrier the terrier, and so on." And again: "The Merino sheep and Heath sheep of Scotland, if two flocks are mixed together, each will breed with its own variety." Mr. Darwin has collected many facts illustrating this point. One of the chief pigeon-fanciers in England informed him that, if free to choose, each breed would prefer pairing with its own kind. Among the wild horses in Paraguay those of the same colour and size associate together; while in Circassia there are three races of horses which have received special names, and which, when living a free life, almost always refuse to mingle and cross, and will even attack one another. On one of the Faroe Islands, not more than half a mile in diameter, the half-wild native black sheep do not readily mix with imported white sheep. In the Forest of Dean, and in the New Forest, the dark and pale coloured herds of fallow deer have never been known to mingle; and even the curious Ancon sheep of quite modern origin have been observed to keep together, separating themselves from the rest of the flock when put into enclosures with other sheep. The same rule applies to birds, for Darwin was informed by the Rev. W.D. Fox that his flocks of white and Chinese geese kept distinct. [Note: some of this is wishful thinking as dog breeds and horse breeds will both mate indiscriminately!]

This constant preference of animals for their like, even in the case of slightly different varieties of the same species, is evidently a fact of great importance in considering the origin of species by natural selection, since it shows us that, so soon as a slight differentiation of form or colour has been effected, isolation will at once arise by the selective association of the animals themselves; and thus the great stumbling-block of "the swamping effects of intercrossing," which has been so prominently brought forward by many naturalists, will be completely obviated.

If now we combine with this fact the correlation of colour with important constitutional peculiarities, and, in some cases, with infertility; and consider, further, the curious parallelism that has been shown to exist between the effects of changed conditions and the intercrossing of varieties in producing either an increase or a decrease of fertility, we shall have obtained, at all events, a starting-point for the production of that infertility which is so characteristic a feature of distinct species when intercrossed. All we need, now, is some means of increasing or accumulating this initial tendency; and to a discussion of this problem we will therefore address ourselves.

THE INFLUENCE OF NATURAL SELECTION UPON STERILITY AND FERTILITY.

It will occur to many persons that, as the infertility or sterility of incipient species would be useful to them when occupying the same or adjacent areas, by neutralising the effects of intercrossing, this infertility might have been increased by the action of natural selection; and this will be thought the more probable if we admit, as we have seen reason to do, that variations in fertility occur, perhaps as frequently as other variations. Mr. Darwin tells us that, at one time, this appeared to him probable, but he found the problem to be one of extreme complexity; and he was also influenced against the view by many considerations which seemed to render such an origin of the sterility or infertility of species when intercrossed very improbable. The fact that species which occupy distinct areas, and which nowhere come in contact with each other, are often sterile when crossed, is one of the difficulties; but this may perhaps be overcome by the consideration that, though now isolated, they may, and often must, have been in contact at their origination. More important is the objection that natural selection could not possibly have produced the difference that often occurs between reciprocal crosses, one of these being sometimes fertile, while the other is sterile. The extremely different amounts of infertility or sterility between different species of the same genus, the infertility often bearing no proportion to the difference between the species crossed, is also an important objection. But none of these objections would have much weight if it could be clearly shown that natural selection is able to increase the infertility variations of incipient species, as it is certainly able to increase and develop all useful variations of form, structure, instincts, or habits. Ample causes of infertility have been shown to exist, in the nature of the organism and the laws of correlation; the agency of natural selection is only needed to accumulate the effects produced by these causes, and to render their final results more uniform and more in accordance with the facts that exist.

About twenty years ago I had much correspondence and discussion with Mr. Darwin on this question. I then believed that I was able to demonstrate the action of natural selection in accumulating infertility; but I could not convince him, owing to the extreme complexity of the process under the conditions which he thought most probable. I have recently returned to the question; and, with the fuller knowledge of the facts of variation we now possess, I think it may be shown that natural selection is, in some probable cases at all events, able to accumulate variations in infertility between incipient species.

The simplest case to consider, will be that in which two forms or varieties of a species, occupying an extensive area, are in process of adaptation to somewhat different modes of life within the same area. If these two forms freely intercross with each other, and produce mongrel offspring which are quite fertile inter se [among themselves], then the further differentiation of the forms into two distinct species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be, perhaps, more vigorous on account of the cross, although less perfectly adapted to the conditions of existence than either of the pure breeds; and this would certainly establish a powerful antagonistic influence to the further differentiation of the two forms.

Now, let us suppose that a partial sterility of the hybrids between the two forms arises, in correlation with the different modes of life and the slight external or internal peculiarities that exist between them, both of which we have seen to be real causes of infertility. The result will be that, even if the hybrids between the two forms are still freely produced, these hybrids will not themselves increase so rapidly as the two pure forms; and as these latter are, by the terms of the problem, better suited to their conditions of life than are the hybrids between them, they will not only increase more rapidly, but will also tend to supplant the hybrids altogether whenever the struggle for existence becomes exceptionally severe. Thus, the more complete the sterility of the hybrids the more rapidly will they die out and leave the two parent forms pure. Hence it will follow that, if there is greater infertility between the two forms in one part of the area than the other, these forms will be kept more pure wherever this greater infertility prevails, will therefore have an advantage at each recurring period of severe struggle for existence, and will thus ultimately supplant the less infertile or completely fertile forms that may exist in other portions of the area. It thus appears that, in such a case as here supposed, natural selection would preserve those portions of the two breeds which were most infertile with each other, or whose hybrid offspring were most infertile; and would, therefore, if variations in fertility continued to arise, tend to increase that infertility. It must particularly be noted that this effect would result, not by the preservation of the infertile variations on account of their infertility, but by the inferiority of the hybrid offspring, both as being fewer in numbers, less able to continue their race, and less adapted to the conditions of existence than either of the pure forms. It is this inferiority of the hybrid offspring that is the essential point; and as the number of these hybrids will be permanently less where the infertility is greatest, therefore those portions of the two forms in which infertility is greatest will have the advantage, and will ultimately survive in the struggle for existence.

The differentiation of the two forms into distinct species, with the increase of infertility between them, would be greatly assisted by two other important factors in the problem. It has already been shown that, with each modification of form and habits, and especially with modifications of colour, there arises a disinclination of the two forms to pair together; and this would produce an amount of isolation which would greatly assist the specialisation of the forms in adaptation to their different conditions of life. Again, evidence has been adduced that change of conditions or of mode of life is a potent cause of disturbance of the reproductive system, and, consequently, of infertility. We may therefore assume that, as the two forms adopted more and more different modes of life, and perhaps acquired also decided peculiarities of form and coloration, the infertility between them would increase or become more general; and as we have seen that every such increase of infertility would give that portion of the species in which it arose an advantage over the remaining portions in which the two varieties were more fertile together, all this induced infertility would maintain itself, and still further increase the general infertility between the two forms of the species.

It follows, then, that specialisation to separate conditions of life, differentiation of external characters, disinclination to cross-unions, and the infertility of the hybrid produce of these unions, would all proceed pari passu [at an equal pace, side by side], and would ultimately lead to the production of two distinct forms having all the characteristics, physiological as well as structural, of true species.

In the case now discussed it has been supposed, that some amount of general infertility might arise in correlation with the different modes of life of two varieties or incipient species. A considerable body of facts already adduced renders it probable that this is the mode in which any widespread infertility would arise; and, if so, it has been shown that, by the influence of natural selection and the known laws which affect varieties, the infertility would be gradually increased. But, if we suppose the infertility to arise sporadically within the two forms, and to affect only a small proportion of the individuals in any area, it will be difficult, if not impossible, to show that such infertility would have any tendency to increase, or would produce any but a prejudicial effect. If, for example, five per cent of each form thus varied so as to be infertile with the other form, the result would be hardly perceptible, because the individuals which formed cross-unions and produced hybrids would constitute a very small portion of the whole species; and the hybrid offspring, being at a disadvantage in the struggle for existence and being themselves infertile, would soon die out, while the much more numerous fertile portion of the two forms would increase rapidly, and furnish a sufficient number of pure-bred offspring of each form to take the place of the somewhat inferior hybrids between them whenever the struggle for existence became severe. We must suppose that the normal fertile forms would transmit their fertility to their progeny, and the few infertile forms their infertility; but the latter would necessarily lose half their proper increase by the sterility of their hybrid offspring whenever they crossed with the other form, and when they bred with their own form the tendency to sterility would die out except in the very minute proportion of the five per cent (one-twentieth) that chance would lead to pair together. Under these circumstances the incipient sterility between the two forms would rapidly be eliminated, and could never rise much above the numbers which were produced by sporadic variation each year.

It was, probably, by a consideration of some such case as this that Mr. Darwin came to the conclusion that infertility arising between incipient species could not be increased by natural selection; and this is the more likely, as he was always disposed to minimise both the frequency and the amount even of structural variations.

We have yet to notice another mode of action of natural selection in favouring and perpetuating any infertility that may arise between two incipient species. If several distinct species are undergoing modification at the same time and in the same area, to adapt them to some new conditions that have arisen there, then any species in which the structural or colour differences that have arisen between it and its varieties or close allies were correlated with infertility of the crosses between them, would have an advantage over the corresponding varieties of other species in which there was no such physiological peculiarity. Thus, incipient species which were infertile together would have an advantage over other incipient species which were fertile, and, whenever the struggle for existence became severe, would prevail over them and take their place. Such infertility, being correlated with constitutional or structural differences, would probably, as already suggested, go on increasing as these differences increased; and thus, by the time the new species became fully differentiated from its parent form (or brother variety) the infertility might have become as well marked as we usually find it to be between distinct species.

This discussion has led us to some conclusions of the greatest importance as bearing on the difficult problem of the cause of the sterility of the hybrids between distinct species. Accepting, as highly probable, the fact of variations in fertility occurring in correlation with variations in habits, colour, or structure, we see, that so long as such variations occurred only sporadically, and affected but a small proportion of the individuals in any area, the infertility could not be increased by natural selection, but would tend to die out almost as fast as it was produced. If, however, it was so closely correlated with physical variations or diverse modes of life as to affect, even in a small degree, a considerable proportion of the individuals of the two forms in definite areas, it would be preserved by natural selection, and the portion of the varying species thus affected would increase at the expense of those portions which were more fertile when crossed. Each further variation towards infertility between the two forms would be again preserved, and thus the incipient infertility of the hybrid offspring might be increased till it became so great as almost to amount to sterility. Yet further, we have seen that if several competing species in the same area were being simultaneously modified, those between whose varieties infertility arose would have an advantage over those whose varieties remained fertile inter se [among themselves], and would ultimately supplant them.

The preceding argument, it will be seen, depends entirely upon the assumption that some amount of infertility characterises the distinct varieties which are in process of differentiation into species; and it may be objected that of such infertility there is no proof. This is admitted; but it is urged that facts have been adduced which render such infertility probable, at least in some cases, and this is all that is required. It is by no means necessary that all varieties should exhibit incipient infertility, but only, some varieties; for we know that, of the innumerable varieties that occur but few become developed into distinct species, and it may be that the absence of infertility, to obviate the effects of intercrossing, is one of the usual causes of their failure. All I have attempted to show is, that when incipient infertility does occur in correlation with other varietal differences, that infertility can be, and in fact must be, increased by natural selection; and this, it appears to me, is a decided step in advance in the solution of the problem.

Footnote to section: As this argument is a rather difficult one to follow, while its theoretical importance is very great, I add here the following briefer exposition of it, in a series of propositions; being, with a few verbal alterations, a copy of what I wrote on the subject about twenty years back. Some readers may find this easier to follow than the fuller discussion in the text: -

Can Sterility of Hybrids have been Produced by Natural Selection?

1. Let there be a species which has varied into two forms each adapted to certain existing conditions better than the parent form, which they soon supplant.

2. If these two forms, which are supposed to coexist in the same district, do not intercross, natural selection will accumulate all favourable variations till they become well suited to their conditions of life, and form two slightly differing species.

3. But if these two forms freely intercross with each other, and produce hybrids, which are also quite fertile inter se [among themselves], then the formation of the two distinct races or species will be retarded, or perhaps entirely prevented; for the offspring of the crossed unions will be more vigorous owing to the cross, although less adapted to their conditions of life than either of the pure breeds.

4. Now, let a partial sterility of the hybrids of some considerable proportion of these two forms arise; and, as this would probably be due to some special conditions of life, we may fairly suppose it to arise in some definite portion of the area occupied by the two forms.

5. The result will be that, in that area, the hybrids (although continually produced by first crosses almost as freely as before) will not themselves increase so rapidly as the two pure forms; and as the two pure forms are, by the terms of the problem, better suited to their several conditions of life than the hybrids, they will inevitably increase more rapidly, and will continually tend to supplant the hybrids altogether at every recurrent severe struggle for existence.

6. We may fairly suppose, also, that as soon as any sterility appears some disinclination to cross unions will appear, and this will further tend to the diminution of the production of hybrids.

7. In the other part of the area, however, where hybridism occurs with perfect freedom, hybrids of various degrees may increase till they equal or even exceed in number the pure species - that is, the incipient species will be liable to be swamped by intercrossing.

8. The first result, then, of a partial sterility of crosses appearing in one part of the area occupied by the two forms, will be - that the great majority of the individuals will there consist of the two pure forms only, while in the remaining part these will be in a minority, - which is the same as saying that the new physiological variety of the two forms will be better suited to the conditions of existence than the remaining portion which has not varied physiologically.

9. But when the struggle for existence becomes severe, that variety which is best adapted to the conditions of existence always supplants that which is imperfectly adapted; therefore, by natural selection the varieties which are sterile when crossed will become established as the only ones.

10. Now let variations in the amount of sterility and in the disinclination to crossed unions continue to occur - also in certain parts of the area: exactly the same result must recur, and the progeny of this new physiological variety will in time occupy the whole area.

11. There is yet another consideration that would facilitate the process. It seems probable that the sterility variations would, to some extent, concur with, and perhaps depend upon, the specific variations; so that, just in proportion as the two forms diverged and became better adapted to the conditions of existence, they would become more sterile when intercrossed. If this were the case, then natural selection would act with double strength; and those which were better adapted to survive both structurally and physiologically would certainly do so.]

PHYSIOLOGICAL SELECTION.

Another form of infertility has been suggested by Professor G.J. Romanes as having aided in bringing about the characteristic infertility or sterility of hybrids. It is founded on the fact, already noticed, that certain individuals of some species possess what may be termed selective sterility - that is, while fertile with some individuals of the species they are sterile with others, and this altogether independently of any differences of form, colour, or structure. The phenomenon, in the only form in which it has been observed, is that of "infertility or absolute sterility between two individuals, each of which is perfectly fertile with all other individuals;" but Mr. Romanes thinks that "it would not be nearly so remarkable, or physiologically improbable, that such incompatibility should run through a whole race or strain."

[A footnote stated: Cases of this kind are referred to at p. 155. It must, however, be noted, that such sterility in first crosses appears to be equally rare between different species of the same genus as between individuals of the same species. Mules and other hybrids are freely produced between very distinct species, but are themselves infertile or quite sterile; and it is this infertility or sterility of the hybrids that is the characteristic - and was once thought to be the criterion - of species, not the sterility of their first crosses. Hence we should not expect to find any constant infertility in the first crosses between the distinct strains or varieties that formed the starting-point of new species, but only a slight amount of infertility in their mongrel offspring. It follows, that Mr. Romanes' theory of Physiological Selection - which assumes sterility or infertility between first crosses as the fundamental fact in the origin of species - does not accord with the general phenomena of hybridism in nature.]

Admitting that this may be so, though we have at present no evidence whatever in support of it, it remains to be considered whether such physiological varieties could maintain themselves, or whether, as in the cases of sporadic infertility already discussed, they would necessarily die out unless correlated with useful characters. Mr. Romanes thinks that they would persist, and urges that "whenever this one kind of variation occurs it cannot escape the preserving agency of physiological selection. Hence, even if it be granted that the variation which affects the reproductive system in this particular way is a variation of comparatively rare occurrence, still, as it must always be preserved whenever it does occur, its influence in the manufacture of specific types must be cumulative." The very positive statements which I have italicised would lead most readers to believe that the alleged fact had been demonstrated by a careful working out of the process in some definite supposed cases. This, however, has nowhere been done in Mr. Romanes' paper; and as it is the vital theoretical point on which any possible value of the new theory rests, and as it appears so opposed to the self-destructive effects of simple infertility, which we have already demonstrated when it occurs between the intermingled portion of two varieties, it must be carefully examined. In doing so, I will suppose that the required variation is not of "rare occurrence," but of considerable amount, and that it appears afresh each year to about the same extent, thus giving the theory every possible advantage.

et us then suppose that a given species consists of 100,000 individuals of each sex, with only the usual amount of fluctuating external variability. Let a physiological variation arise, so that 10 per cent of the whole number - 10,000 individuals of each sex - while remaining fertile inter se [among themselves] become quite sterile with the remaining 90,000. This peculiarity is not correlated with any external differences of form or colour, or with inherent peculiarities of likes or dislikes leading to any choice as to the pairing of the two sets of individuals. We have now to inquire, What would be the result?

Taking, first, the 10,000 pairs of the physiological or abnormal variety, we find that each male of these might pair with any one of the whole 100,000 of the opposite sex. If, therefore, there was nothing to limit their choice to particular individuals of either variety, the probabilities are that 9000 of them would pair with the opposite variety, and only 1000 with their own variety - that is, that 9000 would form sterile unions, and only one thousand would form fertile unions.

Taking, next, the 90,000 normal individuals of either sex, we find, that each male of these has also a choice of 100,000 to pair with. The probabilities are, therefore, that nine-tenths of them - that is, 81,000 - would pair with their normal fellows, while 9000 would pair with the opposite abnormal variety forming the above-mentioned sterile unions.

Now, as the number of individuals forming a species remains constant, generally speaking, from year to year, we shall have next year also 100,000 pairs, of which the two physiological varieties will be in the proportion of eighty-one to one, or 98,780 pairs of the normal variety to 1220 [The exact number is 1219.51, but the fractions were omitted for clearness in the main text] of the abnormal, that being the proportion of the fertile unions of each. In this year we shall find, by the same rule of probabilities, that only 15 males of the abnormal variety will pair with their like and be fertile, the remaining 1205 forming sterile unions with some of the normal variety. The following year the total 100,000 pairs will consist of 99,984 of the normal, and only 16 of the abnormal variety; and the probabilities, of course, are, that the whole of these latter will pair with some of the enormous preponderance of normal individuals, and, their unions being sterile, the physiological variety will become extinct in the third year.

If now in the second and each succeeding year a similar proportion as at first (10 per cent) of the physiological variety is produced afresh from the ranks of the normal variety, the same rate of diminution will go on, and it will be found that, on the most favourable estimate, the physiological variety can never exceed 12,000 to the 88,000 of the normal form of the species, as shown by the following table: -

1st Year. 10,000 of physiological variety to 90,000 of normal variety.
2nd Year. 1,220 + 10,000 again produced.
3rd Year. 16 + 1,220 + 10,000 do.       = 11,236
4th Year. O + 16 + 1,220 + 10,000 do. = 11,236
5th Year. O + 16 + 1,220 + 10,000      = 11,236
and so on for any number of generations.

In the preceding discussion we have given the theory the advantage of the large proportion of 10 per cent of this very exceptional variety arising in its midst year by year, and we have seen that, even under these favourable conditions, it is unable to increase its numbers much above its starting-point, and that it remains wholly dependent on the continued renewal of the variety for its existence beyond a few years. It appears, then, that this form of inter-specific sterility cannot be increased by natural or any other known form of selection, but that it contains within itself its own principle of destruction. If it is proposed to get over the difficulty by postulating a larger percentage of the variety annually arising within the species, we shall not affect the law of decrease until we approach equality in the numbers of the two varieties. But with any such increase of the physiological variety the species itself would inevitably suffer by the large proportion of sterile unions in its midst, and would thus be at a great disadvantage in competition with other species which were fertile throughout. Thus, natural selection will always tend to weed out any species with too great a tendency to sterility among its own members, and will therefore prevent such sterility from becoming the general characteristic of varying species, which this theory demands should be the case.

On the whole, then, it appears clear that no form of infertility or sterility between the individuals of a species, can be increased by natural selection unless correlated with some useful variation, while all infertility not so correlated has a constant tendency to effect its own elimination. But the opposite property, fertility, is of vital importance to every species, and gives the offspring of the individuals which possess it, in consequence of their superior numbers, a greater chance of survival in the battle of life. It is, therefore, directly under the control of natural selection, which acts both by the self-preservation of fertile and the self-destruction of infertile stocks - except always where correlated as above, when they become useful, and therefore subject to be increased by natural selection.

SUMMARY AND CONCLUDING REMARKS ON HYBRIDITY.

The facts which are of the greatest importance to a comprehension of this very difficult subject are those which show the extreme susceptibility of the reproductive system both in plants and animals. We have seen how both these classes of organisms may be rendered infertile, by a change of conditions which does not affect their general health, by captivity, or by too close interbreeding. We have seen, also, that infertility is frequently correlated with a difference of colour, or with other characters; that it is not proportionate to divergence of structure; that it varies in reciprocal crosses between pairs of the same species; while in the cases of dimorphic and trimorphic plants the different crosses between the same pair of individuals may be fertile or sterile at the same time. It appears as if fertility depended on such a delicate adjustment of the male and female elements to each other, that, unless constantly kept up by the preservation of the most fertile individuals, sterility is always liable to arise. This preservation always occurs within the limits of each species, both because fertility is of the highest importance to the continuance of the race, and also because sterility (and to a less extent infertility) is self-destructive as well as injurious to the species.

So long therefore as a species remains undivided, and in occupation of a continuous area, its fertility is kept up by natural selection; but the moment it becomes separated, either by geographical or selective isolation, or by diversity of station or of habits, then, while each portion must be kept fertile inter se [among themselves], there is nothing to prevent infertility arising between the two separated portions. As the two portions will necessarily exist under somewhat different conditions of life, and will usually have acquired some diversity of form and colour - both which circumstances we know to be either the cause of infertility or to be correlated with it, - the fact of some degree of infertility usually appearing between closely allied but locally or physiologically segregated species is exactly what we should expect.

The reason why varieties do not usually exhibit a similar amount of infertility is not difficult to explain. The popular conclusions on this matter have been drawn chiefly from what occurs among domestic animals, and we have seen that the very first essential to their becoming domesticated was that they should continue fertile under changed conditions of life. During the slow process of the formation of new varieties by conscious or unconscious selection, fertility has always been an essential character, and has thus been invariably preserved or increased; while there is some evidence to show that domestication itself tends to increase fertility.

Among plants, wild species and varieties have been more frequently experimented on than among animals, and we accordingly find numerous cases in which distinct species of plants are perfectly fertile when crossed, their hybrid offspring being also fertile inter se [among themselves]. We also find some few examples of the converse fact - varieties of the same species which when crossed are infertile or even sterile.

The idea that either infertility or geographical isolation is absolutely essential to the formation of new species, in order to prevent the swamping effects of intercrossing, has been shown to be unsound, because the varieties or incipient species will, in most cases, be sufficiently isolated by having adopted different habits or by frequenting different stations; while selective association, which is known to be general among distinct varieties or breeds of the same species, will produce an effective isolation even when the two forms occupy the same area.

From the various considerations now adverted to, Mr. Darwin arrived at the conclusion that the sterility or infertility of species with each other, whether manifested in the difficulty of obtaining first crosses between them or in the sterility of the hybrids thus obtained, is not a constant or necessary result of specific difference, but is incidental on unknown peculiarities of the reproductive system. These peculiarities constantly tend to arise under changed conditions owing to the extreme susceptibility of that system, and they are usually correlated with variations of form or of colour. Hence, as fixed differences of form and colour, slowly gained by natural selection in adaptation to changed conditions, are what essentially characterise distinct species, some amount of infertility between species is the usual result.

Here the problem was left by Mr. Darwin; but we have shown that its solution may be carried a step further. If we accept the association of some degree of infertility, however slight, as a not unfrequent accompaniment of the external differences which always arise in a state of nature between varieties and incipient species, it has been shown that natural selection has power to increase that infertility just as it has power to increase other favourable variations. Such an increase of infertility will be beneficial, whenever new species arise in the same area with the parent form; and we thus see how, out of the fluctuating and very unequal amounts of infertility correlated with physical variations, there may have arisen that larger and more constant amount which appears usually to characterise well-marked species.

The great body of facts of which a condensed account has been given in the present chapter, although from an experimental point of view very insufficient, all point to the general conclusion we have now reached, and afford us a not unsatisfactory solution of the great problem of hybridism in relation to the origin of species by means of natural selection. Further experimental research is needed in order to complete the elucidation of the subject; but until these additional facts are forthcoming no new theory seems required for the explanation of the phenomena.

EXCERPTS FROM "THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION" BY CHARLES DARWIN

From Chapter 1.I "Domestic Dogs and Cats"

There is another and closely allied difficulty consequent on the doctrine of the descent of our domestic dogs from several wild species, namely, that they do not seem to be perfectly fertile with their supposed parents. But the experiment has not been quite fairly tried; the Hungarian dog, for instance, which in external appearance so closely resembles the European wolf, ought to be crossed with this wolf: and the pariah dogs of India with Indian wolves and jackals; and so in other cases. That the sterility is very slight between certain dogs and wolves and other Canidae is shown by savages taking the trouble to cross them. Buffon got four successive generations from the wolf and dog, and the mongrels were perfectly fertile together. (1/48. M. Broca has shown ('Journal de Physiologie' tome 2 page 353) that Buffon's experiments have been often misrepresented. Broca has collected (pages 390-395) many facts on the fertility of crossed dogs, wolves, and jackals.) But more lately M. Flourens states positively as the result of his numerous experiments that hybrids from the wolf and dog, crossed inter se [among themselves], become sterile at the third generation, and those from the jackal and dog at the fourth generation. (1/49. 'De la Longevite Humaine' par M. Flourens 1855 page 143. Mr. Blyth says ('Indian Sporting Review' volume 2 page 137) that he has seen in India several hybrids from the pariah-dog and jackal; and between one of these hybrids and a terrier. The experiments of Hunter on the jackal are well-known. See also Isid. Geoffroy St.-Hilaire, 'Hist. Nat. Gen.' tome 3 page 217, who speaks of the hybrid offspring of the jackal as perfectly fertile for three generations.) But these animals were closely confined; and many wild animals, as we shall see in a future chapter, are rendered by confinement in some degree or even utterly sterile. The Dingo, which breeds freely in Australia with our imported dogs, would not breed though repeatedly crossed in the Jardin des Plantes. (1/50. On authority of F. Cuvier quoted in Bronn's 'Geschichte der Natur' b. 2 s. 164.) Some hounds from Central Africa, brought home by Major Denham, never bred in the Town of London (1/51. W.C.L. Martin 'History of the Dog' 1845 page 203. Mr. Philip P. King, after ample opportunities of observation, informs me that the Dingo and European dogs often cross in Australia.); and a similar tendency to sterility might be transmitted to the hybrid offspring of a wild animal. Moreover, it appears that in M. Flourens' experiments the hybrids were closely bred in and in for three or four generations; and this circumstance would most certainly increase the tendency to sterility. Several years ago I saw confined in the Zoological Gardens of London a female hybrid from an English dog and jackal, which even in this the first generation was so sterile that, as I was assured by her keeper, she did not fully exhibit her proper periods; but this case was certainly exceptional, as numerous instances have occurred of fertile hybrids from these two animals. In almost all experiments on the crossing of animals there are so many causes of doubt, that it is extremely difficult to come to any positive conclusion. It would, however, appear, that those who believe that our dogs are descended from several species will have not only to admit that their offspring after a long course of domestication generally lose all tendency to sterility when crossed together; but that between certain breeds of dogs and some of their supposed aboriginal parents a certain degree of sterility has been retained or possibly even acquired.

We have already seen how often savages cross their dogs with wild native species; and Pennant gives a curious account (1/74. 'History of Quadrupeds' 1793 volume 1 page 238.) of the manner in which Fochabers, in Scotland, was stocked "with a multitude of curs of a most wolfish aspect" from a single hybrid-wolf brought into that district.

Several naturalists, as Pallas, Temminck, Blyth, believe that domestic cats are the descendants of several species commingled: it is certain that cats cross readily with various wild species, and it would appear that the character of the domestic breeds has, at least in some cases, been thus affected. Sir W. Jardine has no doubt that, "in the north of Scotland, there has been occasional crossing with our native species (F. sylvestris), and that the result of these crosses has been kept in our houses. I have seen," he adds, "many cats very closely resembling the wild cat, and one or two that could scarcely be distinguished from it." Mr. Blyth (1/89. Asiatic Soc. of Calcutta; Curator's Report, August 1856. The passage from Sir W. Jardine is quoted from this Report. Mr. Blyth, who has especially attended to the wild and domestic cats of India, has given in this Report a very interesting discussion on their origin.) remarks on this passage, "but such cats are never seen in the southern parts of England; still, as compared with any Indian tame cat, the affinity of the ordinary British cat to F. sylvestris is manifest; and due I suspect to frequent intermixture at a time when the tame cat was first introduced into Britain and continued rare, while the wild species was far more abundant than at present." In Hungary, Jeitteles (1/90. 'Fauna Hungariae Sup.' 1862 s. 12.) was assured on trustworthy authority that a wild male cat crossed with a female domestic cat, and that the hybrids long lived in a domesticated state. In Algiers the domestic cat has crossed with the wild cat (F. lybica) of that country. (1/91. Isid. Geoffroy Saint-Hilaire 'Hist. Nat. Gen.' tome 3 page 177.) In South Africa as Mr. E. Layard informs me, the domestic cat intermingles freely with the wild F. caffra; he has seen a pair of hybrids which were quite tame and particularly attached to the lady who brought them up; and Mr. Fry has found that these hybrids are fertile. In India the domestic cat, according to Mr. Blyth, has crossed with four Indian species. With respect to one of these species, F. chaus, an excellent observer, Sir W. Elliot, informs me that he once killed, near Madras, a wild brood, which were evidently hybrids from the domestic cat; these young animals had a thick lynx-like tail and the broad brown bar on the inside of the forearm characteristic of F. chaus. Sir W. Elliot adds that he has often observed this same mark on the forearms of domestic cats in India. Mr. Blyth states that domestic cats coloured nearly like F. chaus, but not resembling that species in shape, abound in Bengal; he adds, "such a colouration is utterly unknown in European cats, and the proper tabby markings (pale streaks on a black ground, peculiarly and symmetrically disposed), so common in English cats, are never seen in those of India." Dr. D. Short has assured Mr. Blyth (1/92. 'Proc. Zoolog. Soc.' 1863 page 184.) that, at Hansi, hybrids between the common cat and F. ornata (or torquata) occur, "and that many of the domestic cats of that part of India were undistinguishable from the wild F. ornata." Azara states, but only on the authority of the inhabitants, that in Paraguay the cat has crossed with two native species. From these several cases we see that in Europe, Asia, Africa, and America, the common cat, which lives a freer life than most other domesticated animals, has crossed with various wild species; and that in some instances the crossing has been sufficiently frequent to affect the character of the breed.

Whether domestic cats have descended from several distinct species, or have only been modified by occasional crosses, their fertility, as far as is known, is unimpaired. The large Angora or Persian cat is the most distinct in structure and habits of all the domestic breeds; and is believed by Pallas, but on no distinct evidence, to be descended from the F. manul of middle Asia; and I am assured by Mr. Blyth that the Angora cat breeds freely with Indian cats, which, as we have already seen, have apparently been much crossed with F. chaus. In England half-bred Angora cats are perfectly fertile with one another.

From Chapter 1.II "Horses and Asses"

I have seen, in the British Museum, a hybrid from the ass and zebra dappled on its hinder quarters.

From Chapter 1.III "Pigs, Cattle, Sheep, Goats"

Bos primigenius and longifrons have been ranked by nearly all palaeontologists as distinct species; and it would not be reasonable to take a different view simply because their domesticated descendants now intercross with the utmost freedom. All the European breeds have so often been crossed both intentionally and unintentionally, that, if any sterility had ensued from such unions, it would certainly have been detected. As zebus inhabit a distant and much hotter region, and as they differ in so many characters from our European cattle, I have taken pains to ascertain whether the two forms are fertile when crossed. The late Lord Powis imported some zebus and crossed them with common cattle in Shropshire; and I was assured by his steward that the cross-bred animals were perfectly fertile with both parent-stocks. Mr. Blyth informs me that in India hybrids, with various proportions of either blood, are quite fertile; and this can hardly fail to be known, for in some districts (3/50. Walther 'Das Rindvieh' 1817 s. 30.) the two species are allowed to breed freely together. Most of the cattle which were first introduced into Tasmania were humped, so that at one time thousands of crossed animals existed there; and Mr. B. O'Neile Wilson, M.A., writes to me from Tasmania that he has never heard of any sterility having been observed. He himself formerly possessed a herd of such crossed cattle, and all were perfectly fertile; so much so, that he cannot remember even a single cow failing to calve. These several facts afford an important confirmation of the Pallasian doctrine that the descendants of species which when first domesticated would if crossed have been in all probability in some degree sterile, become perfectly fertile after a long course of domestication. In a future chapter we shall see that this doctrine throws some light on the difficult subject of Hybridism.

From Chapter 1.IV "Domestic Rabbits"

But from what we hear of the marvellous success in France in rearing hybrids between the hare and rabbit (4/7. See Dr. P. Broca's interesting memoir on this subject in Brown-Sequard 'Journ. de. Phys.' volume 2 page 367.), it is possible, though not probable, from the great difficulty in making the first cross, that some of the larger races, which are coloured like the hare, may have been modified by crosses with this animal. Nevertheless, the chief differences in the skeletons of the several domestic breeds cannot, as we shall presently see, have been derived from a cross with the hare.

From Chapter 2.XVIII. "On The Advantages And Disadvantages Of Changed Conditions Of Life: Sterility From Various Causes"

Many species of Felidae have bred in various menageries, although imported from diverse climates and closely confined. Mr. Bartlett, the present superintendent of the Zoological Gardens (18/17. On the Breeding of the Larger Felidae 'Proc. Zoolog. Soc.' 1861 page 140.) remarks that the lion appears to breed more frequently and to bring forth more young at a birth than any other species of the family. He adds that the tiger has rarely bred; "but there are several well-authenticated instances of the female tiger breeding with the lion." Strange as the fact may appear, many animals under confinement unite with distinct species and produce hybrids quite as freely as, or even more freely than, with their own species.

From Chapter 2.XIII "Inheritance Continued - Reversion or Atavism [Throwbacks]"

Many years ago I saw in the Zoological Gardens a curious triple hybrid, from a bay mare, by a hybrid from a male ass and female zebra [this would have meant a fertile male hybrid]. This animal when old had hardly any stripes; but I was assured by the superintendent, that when young it had shoulder-stripes, and faint stripes on its flanks and legs. I mention this case more especially as an instance of the stripes being much plainer during youth than in old age.

As the zebra has such a conspicuously striped body and legs, it might have been expected that the hybrids from this animal and the common ass would have had their legs in some degree striped; but it appears from the figures given in Dr. Gray's 'Knowsley Gleanings' and still more plainly from that given by Geoffroy and F. Cuvier (13/34. 'Hist. Nat. des Mammiferes' 1820 tome 1), that the legs are much more conspicuously striped than the rest of the body; and this fact is intelligible only on the belief that the ass aids in giving, through the power of reversion, this character to its hybrid offspring.

The quagga is banded over the whole front part of its body like a zebra, but has no stripes on its legs, or mere traces of them. But in the famous hybrid bred by Lord Morton (13/35. 'Philosoph. Transact.' 1821 page 20.) from a chestnut, nearly purely-bred, Arabian mare, by a male quagga, the stripes were "more strongly defined and darker than those on the legs of "the quagga." The mare was subsequently put to a black Arabian horse, and bore two colts, both of which, as formerly stated, were plainly striped on the legs, and one of them likewise had stripes on the neck and body.

The Equus indicus (13/36. Sclater in 'Proc. Zoolog. Soc.' 1862 page 163: this species is the Ghor-Khur of N.W. India, and has often been called the Hemionus of Pallas. See also Mr. Blyth's excellent paper in 'Journal of Asiatic Soc. of Bengal' volume 28 1860 page 229.) is characterised by a spinal stripe, without shoulder or leg stripes; but traces of these latter stripes may occasionally be seen even in the adult (13/37. Another species of wild ass, the true E. hemionus or Kiang, which ordinarily has no shoulder-stripes, is said occasionally to have them; and these, as with the horse and ass, are sometimes double: see Mr. Blyth in the paper just quoted and in 'Indian Sporting Review' 1856 page 320: and Col. Hamilton Smith in 'Nat. Library, Horses' page 318; and 'Dict. Class. d'Hist. Nat.' tome 3 page 563.) and Colonel S. Poole, who has had ample opportunities for observation, informs me that in the foal, when first born, the head and legs are often striped, but the shoulder-stripe is not so distinct as in the domestic ass; all these stripes, excepting that along the spine, soon disappear. Now a hybrid, raised at Knowsley (13/38. Figured in the 'Gleanings from the Knowsley Menageries' by Dr. J.E. Gray.) from a female of this species by a male domestic ass, had all four legs transversely and conspicuously striped, had three short stripes on each shoulder and had even some zebra-like stripes on its face! Dr. Gray informs me that he has seen a second hybrid of the same parentage, similarly striped.

The parents of all our domesticated animals were of course aboriginally wild in disposition; and when a domesticated species is crossed with a distinct species, whether this is a domesticated or only a tamed animal, the hybrids are often wild to such a degree, that the fact is intelligible only on the principle that the cross has caused a partial return to a primitive disposition. Thus, the Earl of Powis formerly imported some thoroughly domesticated humped cattle from India, and crossed them with English breeds, which belong to a distinct species; and his agent remarked to me, without any question having been asked, how oddly wild the cross-bred animals were. The European wild boar and the Chinese domesticated pig are almost certainly specifically distinct: Sir F. Darwin crossed a sow of the latter breed with a wild Alpine boar which had become extremely tame, but the young, though having half-domesticated blood in their veins, were "extremely wild in confinement, and would not eat swill like common English pigs." Captain Hutton, in India, crossed a tame goat with a wild one from the Himalaya, and he remarked to me how surprisingly wild the offspring were.

These latter facts remind us of the statements, so frequently made by travellers in all parts of the world, on the degraded state and savage disposition of crossed races of man. That many excellent and kind-hearted mulattos have existed no one will dispute; and a more mild and gentle set of men could hardly be found than the inhabitants of the island of Chiloe, who consist of Indians commingled with Spaniards in various proportions. On the other hand, many years ago, long before I had thought of the present subject, I was struck with the fact that, in South America, men of complicated descent between Negroes, Indians, and Spaniards, seldom had, whatever the cause might be, a good expression. (13/47. 'Journal of Researches' 1845 page 71.) Livingstone - and a more unimpeachable authority cannot be quoted, - after speaking of a half-caste man on the Zambesi, described by the Portuguese as a rare monster of inhumanity, remarks, "It is unaccountable why half-castes, such as he, are so much more cruel than the Portuguese, but such is undoubtedly the case." An inhabitant remarked to Livingstone, "God made white men, and God made black men, but the Devil made halfcastes." (13/48. 'Expedition to the Zambesi' 1865 pages 25, 150.) When two races, both low in the scale, are crossed the progeny seems to be eminently bad. Thus the noble- hearted Humboldt, who felt no prejudice against the inferior races, speaks in strong terms of the bad and savage disposition of Zambos, or half-castes between Indians and Negroes; and this conclusion has been arrived at by various observers. (13/49. Dr. P. Broca on 'Hybridity in the Genus Homo' English translation 1864 page 39.) From these facts we may perhaps infer that the degraded state of so many half-castes is in part due to reversion to a primitive and savage condition, induced by the act of crossing, even if mainly due to the unfavourable moral conditions under which they are generally reared.

Another form of reversion is far commoner, indeed is almost universal with the offspring from a cross, namely, to the characters proper to either pure parent-form. As a general rule, crossed offspring in the first generation are nearly intermediate between their parents, but the grandchildren and succeeding generations continually revert, in a greater or lesser degree, to one or both of their progenitors. Several authors have maintained that hybrids and mongrels include all the characters of both parents, not fused together, but merely mingled in different proportions in different parts of the body; or, as Naudin (13/50. 'Nouvelles Archives du Museum' tome 1 page 151.) has expressed it, a hybrid is a living mosaic-work, in which the eye cannot distinguish the discordant elements, so completely are they intermingled. [...] Naudin further believes that the segregation of the two specific elements or essences is eminently liable to occur in the male and female reproductive matter; and he thus explains the almost universal tendency to reversion [to one or other parental type] in successive hybrid generations [...] But it would, as I suspect, be more correct to say that the elements of both parent-species exist in every hybrid in a double state, namely, blended together and completely separate. How this is possible, and what the term specific essence or element may be supposed to express, I shall attempt to show in the chapter on the hypothesis of pangenesis.

Naudin's view, as propounded by him, is not applicable to the reappearance of characters lost long ago by variation; and it is hardly applicable to races or species which, after having been crossed at some former period with a distinct form, and having since lost all traces of the cross, nevertheless occasionally yield an individual which reverts (as in the case of the great- great-grandchild of the pointer Sappho) to the crossing form. The most simple case of reversion, namely, of a hybrid or mongrel to its grandparents, is connected by an almost perfect series with the extreme case of a purely-bred race recovering characters which had been lost during many ages; and we are thus led to infer that all the cases must be related by some common bond.

From Chapter 2.XIV "Inheritance Continued. Fixedness Of Character, Prepotency, Sexual Limitation, Correspondence Of Age"

With animals, the jackal is prepotent over the dog, as is stated by Flourens, who made many crosses between these animals; and this was likewise the case with a hybrid which I once saw between a jackal and a terrier. I cannot doubt, from the observations of Colin and others, that the ass is prepotent over the horse; the prepotency in this instance running more strongly through the male than through the female ass; so that the mule resembles the ass more closely than does the hinny. (14/16. Flourens 'Longevite Humaine' page 144 on crossed jackals. With respect to the difference between the mule and the hinny I am aware that this has generally been attributed to the sire and dam transmitting their characters differently; but Colin, who has given in his 'Traite Phys. Comp.' tome 2 pages 537-539, the fullest description which I have met with of these reciprocal hybrids, is strongly of opinion that the ass preponderates in both crosses, but in an unequal degree. This is likewise the conclusion of Flourens, and of Bechstein in his 'Naturgeschichte Deutschlands' b. 1 s. 294. The tail of the hinny is much more like that of the horse than is the tail of the mule, and this is generally accounted for by the males of both species transmitting with greater power this part of their structure; but a compound hybrid which I saw in the Zoological Gardens, from a mare by a hybrid ass- zebra, closely resembled its mother in its tail.)

From Chapter 2.XV "On Crossing"

It is remarkable, as has been strongly insisted upon by Isidore Geoffroy St. Hilaire in regard to animals, that the transmission of characters without fusion occurs very rarely when species are crossed; I know of one exception alone, namely, with the hybrids naturally produced between the common and hooded crow (Corvus corone and cornix), which, however, are closely allied species, differing in nothing except colour. Nor have I met with any well- ascertained cases of transmission of this kind, even when one form is strongly prepotent over another, when two races are crossed which have been slowly formed by man's selection, and therefore resemble to a certain extent natural species. Such cases as puppies in the same litter closely resembling two distinct breeds, are probably due to superfoetation, - that is, to the influence of two fathers.

From Chapter 2.XVIII "On The Advantages And Disadvantages Of Changed Conditions Of Life: Sterility From Various Causes"

The Genetta has bred both here and in the Jardin des Plantes, and produced hybrids.

In the nine-year Report it is stated that the bears had been seen in the Zoological Gardens to couple freely, but previously to 1848 had most rarely conceived. In the Reports published since this date three species have produced young (hybrids in one case) [...]

The common hare when confined has, I believe, never bred in Europe; though, according to a recent statement, it has crossed with the rabbit. (18/21. Although the existence of the Leporides, as described by Dr. Broca ('Journal de Phys.' tome 2 page 370), has been positively denied, yet Dr. Pigeaux ('Annals and Mag. of Nat. Hist.' volume 20 1867 page 75) affirms that the hare and rabbit have produced hybrids.)

A Macacus, according to Flourens, bred in Paris; and more than one species of this genus has produced young in London, especially the Macacus rhesus, which everywhere shows a special capacity to breed under confinement. Hybrids have been produced both in Paris and London from this same genus.

Several members of the family of Lemurs have produced hybrids in the Zoological Gardens.

MULES AND OTHER HYBRIDS.- W. B. Tegetmeier
(The Field, 14th January 1899)

I HAVE READ WITH MUCH INTEREST Mr Morgan Evans's article on " Mules and other Hybrids "in the last number of the Field, and would wish to make some few remarks on the sublet. Mr Evans quotes largely from Mr Romanes's article in the " Encyclopaedia' Britannica” — one which, I have no hesitation in saying, is marked by several very important errors. One of these, the possibility of producing a hybrid between the hare and the rabbit, which is accepted by Mr Romanes, is queried by Mr Evans. There is no doubt whatever that such a hybrid has never been produced. The so-called Leporides we n re simply a variety of rabbit that somewhat resembled the hare in colour. All careful experimenters—such as the late Mr Bartlett of the Zoological Gardens, and others—have found it perfectly impracticable to produce such a hybrid. Moreover, the so-called Leporides have every anatomical character of the domesticated rabbit in the length of the limbs, which are so different in the two animals ; and the blind, naked, and helpless condition in which they are born proves them to be unmistakably rabbits. I visited many years ago the establishment of the late Mr Piney, who was a believer in the existence of this hybrid. On going into his stable I found a number of hutches which could be opened at will by any groom or stable-boy, and which were supposed to contain the parents of this mixed breed. There was a considerable number of hutches, and the experiment was conducted without the slightest attempt at scientific accuracy. The result is not doubtful. The manufacture of Leporides was a fraud perpetrated on the rabbit fanciers, who bought the large animals, which are even now called Belgian Hare rabbits, as Leporides, and found them perfectly fertile. A similar fraud is now going on in the manufacture of varieties in poultry.

Referring to Mr Romanes's article in the Encyclopaedia, I find he states that "it is doubtful whether there is any single instance of a perfectly fertile hybrid having emanated from a cross between two animal species." Mr Romanes was a facile and voluminous writer on these subjects, but was not a practical investigator. The case quoted by Mr Darwin is perfectly well established. The two species of goose, the Chinese and the common, were regarded by Cuvier as belonging to distinct genera; the Chinese goose, with its loud clanging voice, distinctly formed head, and elongated neck, was placed by him amongst the swans and called Cygnus anseroides. These two species breed together, producing hybrids which are perfectly fertile even inter se. The late Mr Blyth informed me that over a large extent of territory he had in some localities no other geese but the hybrid progeny of these two species.

Take again Mr Bartlett's experiments on hybrid bovines. He crossed the bison of North America with a female hybrid bred from the gayal of India and the zebu, so that these hybrids were the result of crossing three distinct genera; nevertheless, they are perfectly fertile. No one could possibly imagine that the wild bison of North America and the gayal of Assam were the same animal. Other examples of the fertility of hybrids could be adduced, and there is no doubt that in some instances they are perfectly fertile, although the great majority are absolutely sterile. Mr Evans in the following paragraph says that the late T. W. Wood believed in the cross between a fowl and a pigeon, and executed a drawing of the supposed hybrid. That Mr Wood drew an accurate drawing of this eccentric little bantam, which was mounted by the taxidermist so as to look as much like a pigeon as possible, I have no doubt, because I utilised his drawing; but I do not think that he believed in the hybridity of the bird any more than I did myself. The distinction between the nesting and rearing of the young of two species is so great that it is impossible to believe in the existence of the hybrid bird, which was nothing more than a mere variety of fowl.

With Mr Evans, I do not believe in the existence of a cross between the sheep and the goat, which is said to be common in America. Both species run semi-feral on the Welsh hills. They are always associated together; but the production of a hybrid in Europe, or, as Mr Evans says, in Africa, is utterly unknown. It is greatly to be regretted that we have not in England, in the Zoological Gardens or elsewhere, any place where experiments or observations on the valuable remits that might attend the hybridisation of on domestic animals might be studied. Mr Bartlett's hybrid bovines should have been utilised. There is no doubt that in the hands of a successful breeder they might, under some conditions or circumstances, have greatly tended to the improvement of our domestic cattle ; but no such experiments were earned out.

The value of horse mules as domestic animals is so great that it is deeply to be regretted that no experiments are made as to the utilisation of the zebras now in the Gardens. I have recently been visiting a large estate where there are some hundreds of first-class horses, and where, at my request, a couple of draught mules were purchased some three or four years since. These were at first hardly received with favour by the stable authorities but they, are now spoken of as being invaluable beasts of burden-docile, quiet, never tired, and doing double the amount of work of any horse of equal size. But the English have always regarded mules with disfavour, although in almost every other civilised country in the world their extreme value is held in the highest estimation. Mr Evans's statement that artificial insemination would soon settle the question of the fertility of mules is one of a very practical character. With regard to the question of the fertility of the Indian mule, whose portrait and that of her supposed progeny was published in the Field, Sept. 17, 1898, I must say that I do not think that the case was proved. There are two explanations possible, one is that the mother is not really a mule, but, like the animal that has been put forth as one in the Acclimatisation Gardens at Paris, is a mare, showing the influence of a first sire ; the other, that if she be a mule she affords an example of induced lactation. Since writing the above I have heard from Mr C. L. Sutherland that the inquiries he has instigated in regard to this case have not yielded satisfactory replies. No evidence short of actual demonstration can be regarded as of an value on the subject, which is so interesting from a physiological, and so important from a practical, point of view that it is, as Mr Evans suggests, worthy of accurate experiment and careful observation. W. B. TEGETMEIER.

ANIMAL LIFE AND THE WORLD OF NATURE
"LION-TIGER HYBRIDS" (June 1902-1903) by A H Bryden

Hybrids from the mating of lion and tiger are by no means common. In the wild state they are unheard of, although in parts of Asia these animals were, and still are, occasionally found in the same haunts; and even in captivity, where cubs from the coupling of the two species have been produced, these mixed offspring have been usually poor specimens, very difficult of rearing. It has remained for one of the most enterprising collectors and naturalists of our time, Mr Carl Hagenbeck, not only to breed, but to bring successfully to a healthy maturity, specimens of this rare alliance between those two great and formidable felidae, the lion and tiger. The illustrations will indicate sufficiently how fortunate Mr Hagenbeck has been in his efforts to produce these hybrids. The oldest and biggest of the animals shown is a hybrid born on the 11th May, 1897. This fine beast, now more than five years old, equals and even excels in his proportions a well-grown lion, measuring as he does from nose tip to tail 10 ft 2 inches in length, and standing only three inches less than 4 ft at the shoulder. A good big lion will weigh about 400 lbs - Mr Selous gives the length of a lion shot by himself as 9 ft 11 inches, its height as 3 ft 8 inches, and its weight as 410 lbs - and it is probably that in very exceptional cases lions may attain 450 and even 500 lbs. But the hybrid in question, weighing as it does no less than 467 lbs, is certainly the superior of most well-grown lions, whether wild-bred or born in a menagerie.

This animal shows faint striping and mottling, and, in its characteristics, exhibits strong traces of both its parents. It has a somewhat lion-like head, and the tail is more like that of a lion than of a tiger. On the other hand, it has little or no trace of mane. It is a huge and very powerful beast, but, like most of Mr Hagenbeck's feline pets, has been reduced to a state of comparative tameness, taking part in the shows given by the keeper, with other felidae, such as lions, tigers, leopards and pumas, not to mention dogs and various species of the genus Ursus.

The next hybrids are a pair born, like the bigger animal, of lion father and tigress mother. They are about seventeen month old, having been born on the 28th April, 1901. They show far more of the tiger striping than their elder relative, and are altogether much more tiger-like in appearance. At the age of twelve or thirteen months - when photographed - they were already as big as most full-grown lions and tigers, and they promise to develop into enormous specimens of the great cat family. They are, apparently, although reduced to a certain state of tameness, none too sweet tempered, judging from their photographs. The small photograph supplied by Mr Hagenbeck shows another pair of hybrid cubs - mere babies, born in April of the present year and photographed a few weeks later. They lie with their foster-mother, a fox terrier, whose services had to be called in for the purpose of rearing them. They too, show strong traces of striping; but as lion cubs in extreme infancy show a good deal of marking also, chiefly spots and blotches, it is possible that these stripes may to some extent disappear as they approach maturity.

These experiments of Mr Hagenbeck are extremely interesting, but, whether fortunately or otherwise, it is almost certain that the breeder of these strange crosses between lion and tiger will never succeed in perpetuating a race of feline mules. Hybrids are notoriously infertile, and although mares - the mules of horses and asses - have strong maternal instincts, and will even develop milk and suckle young foals, which they manage to decoy away from their true mothers, there is, I believe, no genuine recorded instance of such a mule bringing forth offspring of her own. Already, I understand, Mr Hagenbeck has mated the big lion-tiger hybrid with other pure-bred felines, but with no result.

Darwin long since pointed out in his "Origin of Species," that "hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile." That assertion seems to be borne out strongly by the present case, and it seems altogether unlikely that any perpetuation of this new kind of fancy stock is to be looked for. Nor, indeed, is it desirable. In their own wild habitats, and after their own fashion, lions and tigers are necessary and very splendid creatures. A bastard strain from the crossing of these two species is not in the least likely to add to the beauty of the wildernesses of Africa or Asia or to the usefulness of two necessary forms in the scheme of nature. Nor in civilized countries, beyond the mere fact of producing a "sport," or curiosity, are these hybrids likely to be of interest or of use to anyone.

In this case, at all events, nature seems to have wisely set limits which even the ingenuity of man is not likely to be able to evade. The causes of the sterility of first crosses and their hybrid progeny were subjects that puzzled even that profound thinker and enquirer, Darwin. He arrived at the conclusion that such sterility had not been acquired by natural selection. "In the case of hybrids," he says, "it (sterility) apparently depends on their whole organisation having been disturbed by being compounded by two distinct forms; the sterility be closely allied to that which so frequently affects pure species, when exposed to new and unnatural conditions."

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