We may dissent at once from Darwin’s statement which, he thinks, “proves that any amount of modification may be affected by the accumulation of numerous slight variations which are in any way profitable without exercise or habit having been brought into play”; we may dissent if for no other reason than that this begs the whole point at issue, and is not proven. It does not follow because in some colonies all intermediate stages of neuters exist, that in other colonies, where no such intermediate stages are present, these have been slowly weeded out by natural selection, causing to disappear all colonies slightly below the mark. It is this that begs the question. Because we can imagine that intermediate stages between the different castes may have been present, it neither follows that such fluctuating variations have been the basis for the evolution of the more sharply defined types, nor that the imagined advantage of such a change would have led through competition to the extermination of the other colonies. However much we may admire the skill with which Darwin tried to meet this difficulty, let us not put down the results to the good of the theory, but rather repeat once more Darwin’s own words at the end of this chapter, to the effect that the facts do not strengthen the theory.
Sterility between Species
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.
In the first place, he points out that all species are not infertile when crossed with other species. The sterility of various species, when crossed, is so different in degree, and graduates away so insensibly, and the fertility of pure species is so easily affected by various circumstances, that it is most difficult to say where perfect fertility ends and sterility begins. “It can thus be shown that neither sterility nor fertility afford any certain distinction between species and varieties.” Darwin cites several cases in plants in which crosses between species have been successfully accomplished. The following remarkable results are also recorded: “Individual plants in certain species of Lobelia, Verbascum, and Passiflora can easily be fertilized by pollen from a distinct species, but not by pollen from the same plant, though this pollen can be proved to be perfectly sound by fertilizing other plants or species. In the genus Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various orchids as shown by Mr. Scott and Fritz Müller, all the individuals are in this peculiar condition. So that with some species, certain abnormal individuals, and in other species all the individuals, can actually be hybridized much more readily than they can be fertilized by pollen from the same individual plant!”[[14]]
[14]. A somewhat parallel case has recently been discovered by Castle for the hermaphroditic ascidian Ciona intestinalis. In this case the spermatozoa of any individual fail to fertilize the eggs of the same individual, although they will fertilize the eggs of any other individual.
In regard to animals, Darwin concludes that “if the genera of animals are as distinct from each other as are the genera of plants, then we may infer that animals more widely distinct in the scale of nature can be crossed more easily than in the case of plants; but the hybrids themselves are, I think, more sterile.”
The most significant fact in this connection is that the more widely different two species are, so that they are placed in different families, so much the less probable is it that cross-fertilization will produce any result. From this condition of infertility there may be traced a gradation between less different forms of the same genus to almost complete, or even complete, fertility between closely similar species. Darwin further points out that: “The hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced—two classes of facts which are generally confounded together—is by no means strict. There are many cases, in which two pure species, as in the genus Verbascum, can be united with unusual facility, and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.”