152 Darwin, and after Darwin. 



The following is a general summary of Mr. Gulick's 



results : — 



Mr. Wallace's criticism of the theory of Physiological Selec- 

 tion is unsatisfactory ; (i) because he has accepted the funda- 

 mental principle of that theory on pages 173-9. in that he 

 maintains that without the cross-infertility the incipient species 

 there considered would be swamped ; (2) because he assumes 

 that physiological selection pertains simply to the infertility 

 of first crosses, and has nothing to do with the infertility of 

 mongrels and hybrids ; (3) because he assumes that infertility 

 between first crosses is of rare occurrence between species of 

 the same genus, ignor ng the fact that in many species of plants 

 the pollen of the species is pre-potent on the stigma of the same 

 species when it has to compete with the pollen of other species 

 of the same genus ; (4) because he not only ignores Mr. Romanes' 

 statement that cross-infertility often affects "a whole race or 

 strain," but he gratuitously assumes that the theory of Physio- 

 logical Selection excludes this "racial incompatibility" (which 

 Mr. Romanes maintains is the more probable form), and bases his 

 computation on the assumption that the cross-infertility is not 

 associated with any other form of segregation ; (5) because he 

 claims to show that "all infertihty not correlated with some 

 useful variation has a constant tendency to effect its own 

 elimination," while his computation only shows that, if the cross- 

 infertility is not associated with some form of positive segre- 

 gation, it will disappear ' ; and (6) because he does not observe 

 that the positive segregation may be secured by the very form of 

 the physiological incompatibility. . . . Without here entering 

 into any computation, it is evident that, e. g. the prepotency of 

 pollen of each kind with its own kind, if only very slight, will 

 prevent cross-fertilization as effectually as a moderate degree of 

 instinctive preference in the case of an animal. 



' " Positive segregation " is Mr. Gulick's term for forms of homo- 

 gamy other than that which is due to selective fertility. Of these other, 

 or " positive " forms, natural selection is one ; but as it is far from 

 being the only one, the criticism points out that utility is not the 

 only conserving principle with which selective fertiUty may be asso- 

 ciated. 



