Dogs have been frequently crossed with wolves and with jackals, and their hybrid offspring have been found to be fertile inter se 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."[54]
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.
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."[55] 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 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; 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 for eight generations.
Fertility of Hybrids among Plants.
Among plants the cases of fertile hybrids are more numerous, owing, in part, to the large scale on which they are grown by gardeners and nurserymen, and to the greater facility with which experiments can be made. Darwin tells us that Kölreuter found ten cases in which two plants considered by botanists to be distinct species were quite fertile together, and he therefore ranked them all as varieties of each other. In some cases these were grown for six to ten successive generations, but after a time the fertility decreased, as we saw to be the case in animals, and presumably from the same cause, too close interbreeding.