Recent Biology xil 



of 'isolation' — by a social barrier — which were not already 

 recognized and named, yet it is one of the forms which Romanes 

 and Gulick did not recognize nor name. It is also interesting as 

 showing a type of cases in which groups living together (that is, 

 not geographically separated), and at first quite fertile ijiter se, 

 might acquire infertility, as a consequence of other morphologi- 

 cal changes, thus illustrating Darwin's view, but under Romanes' 

 conditions. I have called this choosing a mate under social 

 limitations 'personal selection,'^ but, like all the other 'selec- 

 tions,' it might be scheduled under ' isolation,' of course. 



It is interesting, also, to note that Darwin recognized several 

 forms of isolation (see Romanes' quotation, p. io8, note) besides 

 geographical separation ; and among them two forms which 

 involve physiological selection, i.e., 'breeding at slightly different 

 seasons,' and ' individuals preferring to pair together ' (sexual 

 selection). The latter is a case of physiological selection, if 

 only we make the highly probable assumption that the ' mental 

 preference ' for certain mates carries with it maximum fertility 

 with those mates.^ 



1 In the work, Social and Ethical Interpretations, Sect. 40. See also the 

 table of forms of ' Selection ' given above in this work, Chap. XII. § 2. 



2 This is a correlation which I have never seen suggested anywhere; yet if 

 it should be true, Mr. Wallace would have to admit physiological selection as 

 a sort of organic counterpart of his selective association by recognition marks. 

 Without such a correlation, sexual preference would seem to lose much of its 

 biological significance. It might get some support from the fact that the coy- 

 ness of the female, which, on the hypothesis of Groos {Play of Animals ; see 

 the review of Professor Groos' book following), plays an essential part in 

 sexual selection, demands increased strength and persistence in the male's 

 impulses. It might be made a matter of experiment to determine whether 

 highly coloured, grand-mannered birds are either absolutely or relatively very 

 fertile ; or it might be observed whether sexual-criminals (in whom the im- 

 pulse on the mental side may be considered strong) have unusually large 

 families, or progeny later in life than others — both, however, very complex 

 problems involving other factors. 



