162 Darwin, and after Darwin. 



numerical arguments against or for your theory. Now it seems 

 to me that it will be best to take, in the first instance, the 

 vegetable kingdom, and with regard to it I cannot see how 

 there can be any numerical argument against the theory. For 

 we often have species side by side with others nearly allied, 

 but much more numerous. The condition of these is precisely- 

 analogous to that of your incipient species. They are exposed 

 to fertilization from, say, ten times as numerous individuals of 

 the allied species. They reject this in favour of that from the 

 relatively few individuals of their own. Yet the two species 

 are in competition. I could go through the numerical argu- 

 ments of your assailant word for word, applying them to 

 such a case as this, and they would triumphantly show that 

 the specific fertility of the rarer kind would lead to its certain 

 extinction. Yet we know that this is not so. 



Indeed, the too triumphant character of the logic used against 

 you seems to me to be capable of being turned to your use. 

 If cross-infertility is so intensely disadvantageous to the indi- 

 viduals presenting it, it cannot have been that which made 

 these individuals and their progeny survive. It is therefore 

 a burden which they have carried. But we find that it is 

 more or less present in all the closely allied types that occur 

 on common areas : therefore it must be a necessary feature 

 in the formation of such types ; for it cannot be an accident 

 that it is present in so many. In other words, it must be 

 the price which the individual and his progeny pay for their 

 formation into a type. And this is your theory pure and 

 simple. 



The more I consider the matter, the more I feel that it is 

 impossible to decide as to the sufficiency of selective fertility 

 to explain the formation of species, if we consider merely the 

 effect it would have on the number of individuals, as con- 

 trasted with what it would be if no such peculiarity bad de- 

 veloped itself. Indeed, I may say that on pondering over 

 the matter I have come to the conclusion, that mere fertility 

 is probably a comparatively unimportant factor in the preser- 

 vation of the species, after a certain sufficient degree of fertility 

 is attained. I do not wish to be misunderstood. To a certain 

 point fertility is not only advantageous but necessary, in 



