4i6 THE GRAMMAR OF SCIENCE 



stringent selection would have to take place in each 

 generation. 



Without a barrier to intercrossing during differentia- 

 tion, the origin of species seems inexplicable. Various 

 hypotheses have been suggested, but here, as in so many 

 other branches of the theory of evolution, little has been 

 so far done to determine their quantitative value. For 

 example we have : — 



(i.) Change of period or habit of reproduction. This is 

 really only asserting a differentiation due to direct selection 

 of the reproductive organs, or of their method of functioning, 

 or to an indirect selection owing to the correlation of these 

 organs or characters with other directly selected organs or 

 characters. 



(ii.) Wallace's theory of recognition marks, according 

 to which, it being an advantage to either differentiated 

 type to breed true, certain characteristic marks are 

 developed, or being developed by the principle of 

 correlation are seized upon as recognition marks by the 

 type seeking its like. 



But there are difficulties about this hypothesis, for if 

 A on mating with A' and B' be equally fertile with 

 both, the preference for A', his own type, can arise only 

 from the fact that intercourse with A' requires less exertion, 

 i.e. A' is easier to find, or that intercourse with B' is relatively 

 distasteful, i.e. A' produces more sexual excitement. It is the 

 ease or individual excitement, not the race profit, which 

 guides A's choice, and the utmost that can occur is that 

 the race should profit by a pre-existing sexual tendency. 

 Recognition marks, it seems to me, must follow rather 

 than precede sexual selection. Without assortative mating 

 it is hard to conceive why they should lead a race to 

 breed true. 



More important than these hypotheses is the third one, 

 which I have not hitherto seen noticed : — 



(iii.) Isolation by self- fertilisation and by endogamy.^ 



^ Self-fertilisation may be here taken to include asexual reproduction, and 

 endogamy to include homogamy, both points to be more fully considered in 

 the next chapter. 



