It may be objected that no feeling of incest exists among the lower animals.[1977] According to Mr. Huth, incest “is constantly practised by animals, and habitually by those which are polygamous.”[1978] But, as we have previously seen, among species that live in families, the young, without exception, leave the family as soon as they are able to shift for themselves; and Mr. Huth has adduced not the slightest evidence for his statement that “polygamy among animals means the closest incest.”[1979]
The hypothesis here advocated can, I think, account for all the facts given in the last chapter. It explains how the horror of incest may be independent of experience as well as of education; why the horror of incest refers not only to relations by blood, but very frequently to persons not at all so related; why the prohibitions of consanguineous marriages vary so considerably with regard to the prohibited degrees, applying, however, almost universally to persons who live in the closest contact with each other; and why these prohibitions are so commonly extended much farther on the one side, the paternal or the maternal, than on the other. The question now arises:—How has this instinctive aversion to marriage between persons living closely together originated?
We have seen that a certain degree of similarity as regards the reproductive system of two individuals is required to make their union fertile and the progeny resulting from this union fully capable of propagation. It might, then, be supposed that the highest degree of similarity must be the most beneficial; but in all probability this is not the case. It seems to be necessary not only that the sexual elements which unite shall be somewhat like, but that they shall be in some way different. The similarity must not be too great.
Mr. Darwin, by his careful studies on the effects of cross- and self-fertilization in the vegetable kingdom, contributed more largely than any one else to the discovery of this law. He watched, from germination to maturity, more than a thousand individual plants, produced by crossing and self-fertilization, belonging to fifty-seven species, fifty-two genera, and thirty large families, and including natives of the most various countries.[1980] The result established by this research was, that cross-fertilization is generally beneficial, and self-fertilization injurious; which is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed and self-fertilized flowers, and in the number of seeds produced by the parent-plants.[1981] Hence, whenever plants which are the offspring of self-fertilization are opposed in the struggle for existence to the offspring of cross-fertilization, the latter have the advantage. And this follows, according to Mr. Darwin, from individuals of two distinct kinds having been subjected during previous generations to different conditions, or to their having varied from some unknown cause in a manner commonly called spontaneous, because of that innate tendency to vary and to advance in organization which exists in all beings; so that in either case their sexual elements have been in some degree differentiated.[1982]
As for the animal kingdom, Mr. Darwin remarks that almost all who have bred many kinds of animals, and have written on the subject, have expressed the strongest conviction on the evil effects of close interbreeding.[1983] “Indeed,” says Sir J. Sebright, “I have no doubt but that, by this practice being continued, animals would, in course of time, degenerate to such a degree as to become incapable of breeding at all.... I have tried many experiments by breeding in-and-in upon dogs, fowls, and pigeons; the dogs became, from strong spaniels, weak and diminutive lap-dogs, the fowls became long in the legs, small in the body, and bad breeders.”[1984] Mr. Huth, on the other hand, denies that breeding in-and-in, however close, has proved to be in itself hurtful, and quotes the evidence of numerous breeders whose choicest stocks have always been so bred. But in these cases, as Mr. Wallace remarks, “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.”[1985] The consensus of opinion on this point among eminent breeders is indeed overwhelming, and cannot be reasoned away. According to Crampe’s experiment with the brown rat (Mus decumanus), thirty-nine animals out of 153 born by related parents, i.e., 25·5 per cent., died soon after birth, whereas of 299 animals of parents not related this was the case with twenty-eight only, i.e., 8·4 per cent. The animals of incestuous broods were much smaller and lighter than others, and their fecundity was diminished.[1986] Mr. Huth himself observed, when breeding rabbits in-and-in, that “after the fourth generation there was a diminution of fecundity analogous to the disgust that the stomach would feel at the same diet long continued,” though he found no evil effect in any other way. On the contrary, the in-and-in bred offspring were somewhat heavier than the non-related parent animals.[1987] Professor Preyer has made a similar observation with regard to guinea-pigs: breeding in-and-in produced a considerable loss of fertility, but was accompanied with an increase of weight.[1988] This seems to indicate that the effects of close interbreeding are not always the same.
There are certainly breeders who prefer connecting together the animals nearest allied in blood to one another. But, as Dr. Mitchell observes, “when breeding in-and-in has been practised with so-called good results, the issue is nothing but the development of a saleable defect, which, from the animal’s point of view, must be regarded as wholly unnatural and artificial, and not calculated to promote its well-being or natural usefulness.”[1989]
Many writers suppose that all the evils from close interbreeding depend upon the combination and consequent increase of morbid tendencies common to both parents, the state of whose health decides whether union would be favourable or not to the offspring. “If the parents are perfectly healthy,” says M. Pouchet, “and exempt from all commencing degeneracy, they can only give birth to children at least as healthy as themselves.... But if the same degeneracy has already tainted both the parents, the offspring will show it in a greater degree, and will tend towards entire disappearance.”[1990] The same opinion is held by Sir John Sebright. But being, as an experienced breeder, well aware of the injurious results which almost always follow from interbreeding animals too closely, he adds that, according to his belief, there never did exist an animal without some defect, in constitution, in form, or in some other essential quality, or that at least a tendency to the same imperfection generally prevails in the same family.[1991]
Mr. Darwin, however, has shown it to be highly probable that, though the injury has often partly resulted from the combination of morbid tendencies, the general cause is different. Considering the number of self-fertilized plants that were tried, he thinks it is nothing less than absurd to suppose that in all these cases the mother-plants, though not appearing in any way diseased, were weak or unhealthy in so peculiar a manner that their self-fertilized seedlings, many hundreds in number, were rendered inferior in height, weight, constitutional vigour, and fertility to their crossed offspring.[1992] Moreover, self-fertilization and close interbreeding induce sterility, and this indicates something quite different from the augmentation of morbid tendencies common to both parents.[1993] Hence it seems to be almost beyond doubt that, just as the sterility of distinct species when first crossed, and of their hybrid offspring, depends on their sexual elements having been differentiated in too great a degree, the evils of close interbreeding, or self-fertilization in plants, result chiefly from their sexual elements not having been sufficiently differentiated. But we do not know why a certain amount of differentiation is necessary or favourable for the fertilization or union of two organisms, any more than for the chemical affinity or union of two substances.[1994] It must, however, be observed that no case of complete sterility is met with in self-fertilized seedlings, as is so common with hybrids,[1995] and that interbreeding even of the nearest relations may sometimes, under very favourable circumstances, be continued through several generations without any evil results making their appearance.
It is impossible to believe that a law which holds good for the rest of the animal kingdom, as well as for plants, does not apply to man also. But it is difficult to adduce direct evidence for the evil effects of consanguineous marriages. We cannot expect very conspicuous results from other alliances than those between the nearest relations—between brothers and sisters, parents and children. And the injurious results even of such unions would not necessarily appear at once. Sir J. Sebright remarks that there may be families of domestic animals which go through several generations without sustaining much injury from having been bred in-and-in,[1996] and the offspring of self-fertilized plants do not always show any loss of vigour in the first generations. Man cannot, in this respect, be subjected to experiments like those tried in the case of other animals, and habitual intermarriage of the very nearest relations is, as we have seen, exceedingly rare. Mr. Adam argues that there is no proof of the physical deterioration of those divisions of mankind amongst whom incestuous unions are known more or less to have prevailed—as the Egyptians and Persians.[1997] But among these nations marriage certainly did not always take place between closely related persons; and breeders of domestic animals inform us that the mixing-in even of a drop of unrelated blood is sufficient almost to neutralize the injurious effects of long continued close interbreeding. Again, Mr. Huth asserts that, though the Ptolemies habitually married their sisters, nieces, and cousins, they were neither sterile nor particularly short-lived.[1998] Mr. Galton, on the contrary, sees in Ptolemaic experience a proof that close intermarriage is followed by sterility.[1999] In ten marriages between brothers and sisters, uncles and nieces, or between first-cousins, the average number of children was not quite two, and three of the unions were entirely sterile.[2000]