1897] REPRODUCTIVE DIVERGENCE 187 



frequent operation in those species having a tendency to split up 

 into varieties, it may be concluded that it was, and is, an extremely 

 important factor in the production of sterility of crosses between 

 species. Thus, as has already been mentioned, it is not supposed 

 that reproductive divergence comes into effect in fixed and stable 

 species, but only in those which, probably in consequence of changes 

 of conditions of environment, are in the course of splitting up into 

 varieties and new species. 



Connected with the fact of the general mutual fertility of varie- 

 ties, is that of the swamping effects of intercrossing. Thus if 

 varieties are perfectly fertile with the parent form, it is difficult to 

 see how they can ever establish themselves as incipient species, 

 unless they become separated from the parent form by a geographi- 

 cal or other barrier. If, however, these varieties have arisen in 

 consequence of the operation of reproductive divergence, it is obvious 

 that they can preserve their characteristics unobliterated, and con- 

 tinue to exist in the same region as the parent form. 



One of the most important objections to the doctrine of the all- 

 sufficiency of Natural Selection as a cause of evolution, is that of 

 the very frequent inutility of specific characters. Some naturalists, 

 especially Wallace, are inclined to maintain that all specific charac- 

 ters are of use, and that it is only due to our ignorance that they 

 appear to us useless. It is a more generally received opinion, on 

 the other hand, that these characters can frequently be of no useful 

 purpose to the organism, and must therefore have originated by some 

 other means than Natural Selection. Darwin himself was fully 

 alive to this objection, and considered that such useless specific 

 characters might owe their origin to the correlation of organs, or to 

 the laws of growth, and to so-called spontaneous variability. 1 These 

 seem but very inefficient causes for such frequently occurring effects, 

 and hence there is a strong prima facie evidence that some other 

 principle is at work. The principle of reproductive divergence offers 

 a satisfactory solution of the problem. Thus, as we have seen, by 

 means of it species are caused to diverge in respect of one or more 

 characteristics, and so fresh or altered characteristics can be originated 

 without the influence of Natural Selection. To take a concrete in- 

 stance, one mentioned by Bateson. 2 The commonest forms of lady- 

 birds are the small Coccinella dcccmpunctata, and the larger C. 

 scptempundata. The small insect is very variable in colour and 

 the pattern of its colours, whilst the large is almost absolutely con- 

 stant in these respects. This difference in specific characters may 

 have originated by a common parent form having had its colour 

 marking, and also the size of the individuals to a slight extent corre- 



1 "Origin of Species," 6tli ed., p. 171. 



- " Materials for the Study of Variation," p. 572. 



