EVOLUTION 425 



unions, they wonderfully emphasise the comparative ease 

 with which selection can so differentiate the reproductive 

 organs that fertilisation is mechanically or physiologically 

 impossible/ I shall content myself here with some illustra- 

 tions of how exact quantitative methods can be applied to 

 the problems of apolegamic and homogamic mating. 



S 2. — Preferential Mating 



If we wish to discover whether preferential mating 

 with regard to any organ or character is taking place in 

 a given form of life, we must investigate whether the type 

 and variability of the mated and unmated members of 

 one or other sex are the same. If they are not, then 

 sexual selection in the form of preferential mating is 

 undoubtedly at work. But in this matter we shall find 

 that sexual selection is just as hard to deal with as 

 natural selection (p. 406). We cannot be certain that 

 the organ discussed is the one directly preferred, its type 

 may have been modified owing to the selection of a 

 correlated organ. For example, let us suppose that the 

 mean eye-colour of wives differs from that of spinsters of 

 middle age. Are we to conclude that this is directly 

 due to a selection of wives by eye-colour, or is indirectly 

 due to the fact that eye-colour is correlated with hair- 

 colour, complexion, and even stature ? At most we can 

 only make a plausible guess, and it would be safest merely 

 to affirm the existence of preferential mating without 

 specification of characters or organs. 



Again, we must be careful to take our mated and 

 unmated material homogeneous. We are almost certain, 

 for example, to find a change of type and variability if 

 we compare parent and offspring. In the first place, 

 natural selection and even growth may act periodically 

 on offspring before they become possible mates. In the 



1 Darwin himself writes : "Bearing in mind what has just been said on 

 the extreme sensitiveness and delicate affinities of the reproductive system, 

 why should we feel any surprise at the sexual elements of those forms, which 

 we call species, having been differentiated in such a manner that they are 

 incapable, or only feebly capable of acting on one another?" — Cross aud Self- 

 fertilisation, p. 473. 



