"Evidence of the evil effects of close interbreeding can most readily be acquired in the case of animals, such as fowls, pigeons, etc., which propagate quickly, and, from being kept in the same place, are exposed to the same conditions. Now, I have inquired of very many breeders of these birds, and I have hitherto not met with a single man who was not thoroughly convinced that an occasional cross with another strain of the same sub-variety was absolutely necessary. Most breeders of highly improved or fancy birds value their own strain, and are most unwilling, at the risk, in their opinion, of deterioration, to make a cross. The purchase of a first-rate bird of another strain is expensive, and exchanges are troublesome; yet all breeders, as far as I can hear, excepting those who keep large stocks at different places for the sake of crossing, are driven after a time to take this step." (P. 117, vol. ii.)
And again, on page 125, he says: "With pigeons, breeders are unanimous, as previously stated, that it is absolutely indispensable, notwithstanding the trouble and expense thus caused, occasionally to cross their much-prized birds with individuals of another strain, but belonging, of course, to the same variety." He then dwells at some length upon the great delicacy of constitution entailed by the close interbreeding of nearly-related pigeons, and mentions a circumstance for which the reason is at once obvious upon our theory. He says, "It deserves notice that, when large size is one of the desired characters, as with pouters, the evil effects of close interbreeding are much sooner perceived than when small birds, such as short-faced tumblers, are desired."
"In the case of the fowl," says Darwin, "a whole array of authorities could be given against too close interbreeding." (P. 124, vol. ii.) Following this assertion is mention of the great sterility of bantams, induced by close interbreeding. He assures us that he has seen silver bantams almost as barren as hybrids. The Sebright bantam is destitute of hackles and sickle tail-feathers. This involves disproportionate development; and that the evil is attributable to this, Darwin virtually admits when he says, on page 101, that the loss of fertility is to be ascribed "either to long-continued, close interbreeding, or to an innate tendency to sterility correlated with the absence of hackles and sickle tail-feathers."
Of all the phenomena attendant upon close interbreeding, we know of none which so strikingly confirms our view as the following curious case. It is a most delicate exemplification of our doctrine. "Mr. Hewitt says that with these bantams the sterility of the male stands, with rare exceptions, in the closest relation with their loss of certain secondary male characters;" he adds, "I have noticed, as a general rule, that even the slightest deviation from feminine character in the tail of the male Sebright—say the elongation by only half an inch of the two principal tail-feathers—brings with it improved probability of increased fertility." (Pp. 124.) The full significance of this singular fact the reader will at once appreciate. For the cause of the phenomenon is obvious. The increased probability of fertility, consequent on the growth of the secondary sexual characters, is owing to the induced return to proportionate development.
Darwin says, "There is reason to believe, and this was the opinion of that most experienced observer, Sir J. Sebright, that the evil effects of close interbreeding may be checked by the related individuals being separated during a few generations and exposed to different conditions of life." (Pp. 115.) Now, different conditions are, as we have seen, favorable to the development of different parts. Exposure, then, to conditions other than those to which their brothers are subjected, would lead to the growth or strengthening of certain parts in the separated animals. Interbreeding between members of the two lots of animals would, in consequence, be equivalent to crossing. The check to the evil effects is to be attributed to a slight dissimilarity of structure.
These quotations from Darwin place beyond doubt the fact that the greatest evil effects flow from the close interbreeding of fowls and pigeons. It now remains for us to show that, in animals which are comparatively proportionately developed, the evil effects are very small. It must be observed that it does not rest with us to show a total absence of evil. For no animals are, in all respects, proportionately developed. Our very ability to discriminate between different breeds necessarily implies the disproportionate development of all but one of them; that is, when their differences are not merely those of size. With cows, want of proportion is often caused by blind conformity in certain breeds to certain standards. Thus, when a breed acquires a reputation, all its points are faithfully preserved, as if the preservation intact of the existing condition of all the features was a sine qua non of the animal's good quality; and this occurs even when some of the features are shockingly out of proportion, or greatly reduced. If one breed were fully and proportionately developed, the others could be distinguished from it only by negative features.
Of the close interbreeding of the cow Darwin says:
"With cattle there can be no doubt that extremely close interbreeding may be long carried on, advantageously with respect to external characters and with no manifestly apparent evil as far as constitution is concerned. The same remark is applicable to sheep. Whether these animals have been rendered less susceptible than others to this evil, in order to permit them to live in herds—a habit which leads the old and vigorous males to expel all intruders, and in consequence often to pair with their own daughters—I will not pretend to decide. The case of Bakewell's longhorns, which were closely interbred for a long period, has often been quoted; yet Youatt says the breed 'had acquired a delicacy of constitution inconsistent with common management,' and 'the propagation of the species was not always certain.' But the shorthorns offer the most striking case of close interbreeding; for instance, the famous bull Favorite (who was himself the offspring of a half-brother and sister from Foljambe) was matched with his own daughter, granddaughter, and great-granddaughter; so that the produce of this last union, or the great-great-granddaughter, had fifteen sixteenths, or 93.75 per cent, of the blood of Favorite in her reins. This cow was matched with the bull Wellington, having 62.5 per cent of Favorite blood in his veins, and produced Clarissa; Clarissa was matched with the bull Lancaster, having 68.75 of the same blood, and she yielded valuable offspring. Nevertheless, Collings, who reared these animals, and was a strong advocate for close interbreeding, once crossed his stock with a Galloway, and the cows from this cross realized the highest prices. Bates's herd was esteemed the most celebrated in the world. For thirteen years he bred most closely in-and-in; but during the next seventeen years, though he had the most exalted notion of the value of his own stock, he thrice infused fresh blood into his herd; it is said that he did this, not to improve the form of his animals, but on account of their lessened fertility. Mr. Bates's own view, as given by a celebrated breeder, was, that 'to breed in-and-in from a bad stock was ruin and devastation; yet that the practice may be safely allowed within certain limits when the parents so related are descended from first-rate animals.' We thus see that there has been extremely close interbreeding with shorthorns; but Nathusius, after the most careful study of their pedigrees, says that he can find no instance of a breeder who has strictly followed this practice during his whole life. From this study and his own experience, he concludes that close interbreeding is necessary to ennoble the stock; but that in effecting this the greatest care is necessary on account of the tendency to infertility and weakness. It may be added that another high authority asserts that many more calves are born cripples from shorthorns than from any other and less closely interbred races of cattle." (Pp. 117, 118, vol. ii.)
This last phenomenon is doubtless due to correlation between the legs and the small development of the horns.
Now, these remarks of Mr. Darwin unequivocally show that extremely long-continued close interbreeding is possible with cattle. They also acquaint us with the fact that, although this may long be carried on, evil at length begins to manifest itself. This is easily explained. A small want of proportion in the animals interbred entails evil, but evil too small in amount to be capable of manifesting itself at once. But continued exacerbations, consequent on frequent pairing with related individuals possessing an evil identical in kind, so augments the evil as eventually to involve its display.