Appendix A. 155 



and so are eventually preserved as permanently distinct forms, 

 is no other than that which Mr. Romanes and myself have 

 discussed under the terms Physiological Selection and Segregate 

 Fecundity. Not only is Mr. Wallace's exposition of the diverg- 

 ence and the continuance of the same in accord with these 

 principles which he has elsewhere rejected, but his whole 

 exposition is at variance with his own principle, which, in the 

 previous chapter, he vigorously maintains in opposition to my 

 statement that many varieties and species of Sandwich Island 

 land molluscs have arisen, while exposed to the same environment, 

 in the isolated groves of the successive valleys of the same 

 mountain range. If he adhered to his own theory, "the greater 

 infertility between the two forms in one portion of the area" 

 would be attributed to a difference between the environment pre- 

 sented in that portion and that presented in the other portions ; 

 and the difficulty would be to consistently show how this 

 greater infertility could continue unabated when the varieties 

 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. Therefore, in 

 order to solve the problem of the origin and increase of 

 infertility between species, he tacitly gives up his own theory, 

 and adopts not only the theory of Physiological Selection but 

 that of Intensive Segregation^ through Isolation, though he 

 still insists on calling the process natural selection ; for on 

 page 183 he says, "No form of infertility or sterility between 

 the individuals of a species can be increased by natural selec- 

 tion unless correlated with some useful variation, while all 

 infertility not so correlated 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 

 species] becomes separated either by geographical or selective 

 isolation, or by diversity 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." 



' By Intensive Segregation Mr. GuHck means what I have called Inde- 

 pendent Variability. 



