Gulick — Cross-infertility. 439 



would be attributed to a difference between the environment 

 presented in that portion and that presented in the other por- 

 tions ; and the difficulty would be to consistently show how 

 this greater infertility could continue unabated when the varie- 

 ties thus characterized spread beyond the environment on 

 which the character depends. But, without power to continue, 

 the process which he describes would not take place. In order 

 to solve the problem of the origin and increase of infertility 

 between species he gives up his own theory and adopts not 

 only the theory of Physiological Selection but that of Inten- 

 sive Segregation through Isolation, though he still insists on 

 calling the process natural selection ; for on page 183 he says, 

 " ISTo form of infertility or sterility between the individuals of 

 a species can be increased by natural selection unless correlated 

 with some useful variation, while all infertility not so correla- 

 ted has a constant tendency to effect its own elimination." 

 Even this claim he seems to unwittingly abandon when on 

 page 184 he says : " The moment it [a speciesj becomes separa- 

 ted either by geographical or selective isolation, or by diver- 

 sity of station or of habits, then, while each portion must be 

 kept fertile inter se, there is nothing to prevent infertility 

 arising between the two separated portions." 



Mr. Wallace adopts these two fundamental doctrines of the 

 theory of Divergent Evolution through Segregation, but he 

 does not apply them exactly as I would. Why, for example, 

 should he resort to the supposition that when the two diver- 

 gent varieties occupying the extensive area are everywhere 

 somewhat infertile with each other the increase of that charac- 

 ter is gained only in a limited portion of the area, and then 

 spreads by conquest ? Would it not be simpler, and at the 

 same time truer to the facts of nature, to assume, that the 

 divergent variety can not arise except as it is aided by some 

 form of positive segregation, preventing free crossing with the 

 parent stock ; and that in cases where this prevention is only 

 partial, any cross-infertility once introduced will diminish the 

 swamping effect of the crossing that occurs, and will every- 

 where tend to increase, because the majority of each generation 

 of the pure form will be the descendants of those whose cross- 

 infertility was above the average. There are, moreover, other 

 forms of negative segregation equally effective with cross- 

 infertility and Segregate Fecundity. It often occurs that when 

 segregation with divergence has once begun to show itself, the 

 variations that are most fully endowed with the new character, 

 (though less adapted to the new mode of life than the old 

 form is to the old mode of life) will best escape both the severe 

 competition with the rest of the species and the swamping 

 effect of crossing. In time the pressure for food becomes as 



