438 Gidick — Cross-infertility. 



the illustrative case introduced by him, the commencement of 

 the cross-infertility is in the relations to each other of two 

 portions of the species partially segregated from the rest by 

 occupying a definite part of the general area, and partially 

 segregated from each other by different modes of life. These 

 two physiologically segregated local varieties, being, by the 

 terms of his supposition, better adapted to the environment 

 than the more freely interbreeding forms in the other parts of 

 the general area, increase till they supplant these original 

 forms. Then, in some limited portion of the general area, 

 there arise two still more divergent varieties, with greater 

 mutual infertility, and therefore with still less commingling of 

 the two, and with power to prevail throughout the whole 

 area.* 



The process here described, if it takes place, is not modifica- 

 tion by natural selection, but a supplanting which does not 

 produce modification, and which does not take place till a new 

 and complicated adjustment has arisen in a portion of the 

 species that is partially segregated, by occupying a definite 

 portion of the area. This new adjustment introduces two new 

 varieties, each with unabated fertility with the other variety ; 

 and the process, or principle, by which it is reached receives 

 no explanation in the section we are now considering ; but. 

 from what he says on page 184, we may judge that his only 

 explanation is an application of the principle of Intensive Segre- 

 gation, more especially that form of this principle which I 

 have described as the effect of isolation on unstable adjust- 

 ments, but which Mr. Wallace has rejected as untenable. 

 Moreover, in the supposed case pictured by Mr. Wallace, the 

 principle, by which the two forms are kept from crossing and 

 are 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 divergence and the 

 continuance of the same in accord with these principles which 

 he has elsewhere rejected, but his whole exposition is at vari- 

 ance 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 infer- 

 tility between the two forms in one portion of the area"' 



* This brief outline of the method by which Mr. Wallace thinks cross-infertility 

 has been produced and accumulated, though given, in another connection, in my 

 article on Utilitarianism as the Exclusive Theory of Organic Evolution (this Jour- 

 nal, July, 1890), is here repeated, that the correspondences and divergences in the 

 different theories we are here discussing may be better apprehended. 



