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, igm r 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 infertility 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 pomts out that utility is not the 

 only conserving principle with which selective fertility may be asso- 

 ciated. 



