Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/441

425 HYBRIDISM 425 great deal of evidence to indicate that prolonged domesti cation has a tendency to eliminate sterility ; so that wild species which when first domesticated intercross with difficulty, become in time able to intercross with facility. Such, for instance, appears to have been the case with the dog ; for on the one hand all the domestic varieties of this animal are now freely fertile among themselves, and, on the other hand, there is independent evidence that these varieties have sprung from more than one natural species. Again, mention must not be omitted of the important fact that, although in the case of none of our varieties of domesticated animals is there any evidence of mutual sterility, yet among our varieties of domesticated plants a few cases have been observed of complete mutual sterility, which is iu every way analogous to that which occurs between natural species. Thus, Gartner observed this to be the case with certain varieties of maize and Verbascum, Kolreuter with one kind of tobacco, and other experiment alists with sundry varieties of gourd ancl melon. And here, let it be observed, we have the exact counterpart that evolutionists would desire to the experiments of Herbert above mentioned ; for while he was able to break down the general distinction between natural species and domestic varieties on the side of proving perfect fertility between certain natural species, these experimentalists have broken down the distinction on the side of proving perfect sterility between certain of our domestic varieties. Therefore, we may conclude that this side of the question, or that as to the fertility of our domestic varieties, presents as harmless an aspect towards the theory of descent as we have already seen to be presented by the other side of the question, or that as to the sterility, of natural species. Finally, there are two complementary considerations to be adduced, which may now be stated together. One is that the general principles of hybridism, as briefly stated at the beginning of this article, are really far from indicative of having been instituted with any design of simply prevent ing species from intercrossing. For, upon the view that they were so instituted, scarcely any one of them admits of a rational explanation. Thus, upon this view, no reason can be assigned why the degree of sterility should be so extremely variable in different species, when it must be supposed equally important that all species should be kept distinct ; nor can it be said why the degree of sterility should vary even among individuals of the same species. Neither can it be said why some species should cross with facility, and yet produce sterile hybrids, while other species cross with difficulty, and yet produce fertile hybrids. Why should species living in countries remote from one another, and therefore not able in a state of nature to come together, nevertheless prove as sterile inter se as species inhabiting the same country? Why, again, should there often be so greit a difference in the result of a reciprocal cross between two species ? Or why, indeed, should the production of hybrids have been permitted at all? As Mr Darwin ob serves, &quot;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 imion between their parents, seems a strange arrangement.&quot; The other and complementary consideration which has to be mentioned is that, on the counter supposition of all these general principles of hybridism being &quot;simply inci dental,&quot; or dependent on unknown differences in the repro ductive systems of species, on this supposition we meet with sundry differences between wild species and domestic varieties which are fully analogous to their difference of fertility, and which yet cannot reasonably be supposed to serve any transcendental purpose. Thus, again to quote Mr Darwiu : &quot;Some allied species of trees cannot be grafted on each other all varieties can be so grafted. Some allied animals are affected in a very different manner by the same poison, but with varieties no such case until recently was known, but now it has been proved that immunity from certain poisons stands in some cases in correla tion with the colour of the hair. The period of gestation generally differs much with distinct species, but with varieties until lately no such difference had been observed. The time required for the ger mination of seeds differs in an analogous manner, and I am not aware that any difference in this respect has as yet been detected with varieties. Here we have various physiological differences, and no doubt others could be added, between one species and another of the same genus, which do not occur, or occur with extreme rarity, in the case of varieties ; and these differences are apparently wholly or in chief part incidental on other constitutional differences, just in the same manner as the sterility of crossed species is incidental on differences confined to the sexual system. Why, then, should these latter differences, however serviceable they may indirectly be in keeping the inhabitants of the same country distinct, be thought of such paramount importance in comparison with other incidental and functional differences ? Xo sufficient answer to this question can be given.&quot; Upon the whole, therefore, it may be concluded that the difficulty which the facts of hybridism seem at first sight to raise against the theory of descent may be explained ir harmony with the main requirements of that theory. Animal Hybrids.- A few words may here be added with special reference to animal hybrids. As a general statement it may be said that hybrids, not only between specific but generic forms, are more easily produced in the cise of animals than in that of plants. The hybrids, how ever, when produced are, as a general rule, more sterile. Indeed, it is doubtful whether there is any single instance of a perfectly fertile hybrid having emanated from a cross between two animal species. Mr Darwin, however, says &quot; I have reason to believe that the hybrids from Cervuhis vaginalis and Reevesii, and from Phasianus colchicus with P. torquatns, are perfectly fertile.&quot; Also M. Quatrefages states that the hybrid progeny of two moths (Boinbyx cynthia and B. arrindia) showed themselves to be fertile inter se for eight generations. The hare and rabbit are said occasionally to breed together, and their offspring to be highly fertile when crossed with either parent species. Lastly, Mr Darwin observes : &quot; The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they have sometimes been ranked in distinct genera, have often bred in this country with either pure parent, and in several instances inter se. This was effected by Mr Eyton, who raised two hybrids from the same parents, but from differ ent hatches ; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far more fertile ; for I am assured by two eminently capable judges, namely, Mr Blyth and Captain Hutton, that whole flocks of these crossed geese are kept in various parts of the country ; and, as they are kept for profit where neither pure parent-species exists, they must certainly be highly or perfectly fertile. &quot; 1 It is somewhat remarkable that hitherto direct experi ments on the hybridization of animals have been so few in number as compared with those on the hybridization of plants. This is the more to be regretted, because, as already observed, animals appear to display a somewhat greater aptitude for hybridizing than plants, and conse quently furnish better material for ascertaining the furth :,t limits of systematic affinity within which a cross may prove fertile. But here direct experiments are wanting, and all we can say with certainty is that in animals, as in plants, no authentic instance is on record of progeny resulting from a union of two individuals separated from one another by more than a generic distinction. 1 Since the above vas published, Mr Darwin has himself procured two of these hybrid geese, and from them (brother and sister) raised five &quot; extremely fine birds from two hatches.&quot; These five hybrids &quot; re sembled in every detail their hybrid parents&quot; (see Mature, Jan. 1, 1880). XII. 54