The care with which Darwin examined every bearing of his theory is nowhere better exemplified than in his treatment of the question of sterility between the individuals of different species. It would be so obviously to the advantage of the selection theory if it were true that sterility between species had been acquired by selection in order to prevent intercrossing, that it would have been easy for a less cautious thinker to have fallen into the error of supposing that sterility might have been acquired in this way. Tempting as such a view appears, Darwin was not caught by the specious argument, as the opening sentence in the chapter of hybridism shows:— “The view commonly entertained by naturalists is that species, when intercrossed, have been specially endowed with sterility, in order to prevent their confusion. This view certainly seems at first highly probable, for species living together could hardly have been kept distinct had they been capable of freely crossing. The subject is in many ways important for us, more especially as the sterility of species when first crossed, and that of their hybrid offspring, cannot have been acquired, as I shall show, by the preservation of successive profitable degrees of sterility. It is an incidental result of differences in the reproductive systems of the parent species.” In dealing with this subject Darwin points out that we must be careful to distinguish between “the sterility of species when first crossed, and the sterility of hybrids produced from them.” In the former case, the reproductive organs of each individual are in a perfectly normal condition, while hybrids appear to be generally impotent owing to some imperfection in the reproductive organs themselves. They are not perfectly fertile, as a rule, either with each other, or with either of the parent forms. In striking contrast to the sterility between species is the fertility of varieties. If, as Darwin believes, varieties are incipient species, we should certainly expect to find them becoming less and less fertile with other fraternal varieties, or with the parent forms in proportion as they become more different. Yet experience appears to teach exactly the opposite; but the question is not a simple one, and the results are not so conclusive as appears at first sight. Let us first see how Darwin met this obvious contradiction to his view.