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.