154 Darwin, and after Darwin. 



mice ; but this kind of selection modifies neither rats nor mice. 

 On the other hand, if certain variations of mice prevail over 

 others, through their superior success in escaping their pur- 

 suers, then modification begins. Now, turning to page 175, we 

 find that, in the illustrative case introduced by Mr. Wallace, 

 the commencement of infertility between the incipient species 

 is in the relations to each other of two portions of a species 

 that are locally segregated from the rest of the species, and 

 partially segregated from each other by different modes of 

 life. These two local varieties, being by the terms of his 

 supposition better adapted to the environment than the freely 

 interbreeding forms in 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 

 forms, with greater mutual infertility, and with increased adap- 

 tation to the environment, enabling them to prevail throughout 

 the whole area. The process here described, if it takes place, 

 is not modification by natural selection. 



On the other hand, it is modification by physiological 

 selection. For, among the several other forms of isolation 

 which are called into requisition, the physiological (i.e. 

 ever accumulating cross- infertility) is supposed to play an 

 important part. That the modification is not modification 

 by natural selection may perhaps be rendered more 

 apparent by observing, that in as far as any other mode 

 of isolation is involved or supposed, so far is the possible 

 agency of natural selection eliminated as between the two 

 or more otherwise isolated sections of a species ; and yet it is 

 modes of isolation other than that furnished by natural selec- 

 tion (i.e. perishing of the less fit), that Mr. Wallace here 

 supposes to have been concerned including, as I have 

 before shown, the physiological form, to which, indeed, he 

 really assigns most importance of all. Or, as Mr. Gulick 

 states the matter in his independent criticism: 



In the supposed case pictured by Mr. Wallace, the principle 

 by which the two segregating forms are kept from crossing, 



