114 CALIFORNIA ACADEMY OF SCIENCES. 



The second difficulty of the doctrine of natural selec- 

 tion as the originator of species, according to Mr. Ro- 

 manes, is the swamping effect of interbreeding. 

 AVherever any geographical barrier cuts off a portion of 

 a race the section so isolated is found to develop into a 

 new species. In fact, it is evident that if occasional 

 variations arise in individuals in the midst of others in 

 which they do not occur, they will soon be eradicated 

 by interbreeding with the dominant class, however ad- 

 vantageous they might be. Physiological selection 

 would of course prevent this. 



Mr. Romanes' third difficulty is a question of fact. 

 Are all specific characters useful? Natural selection 

 concerns itself solely with adaptations caring nothing 

 whatever for species as such. In case the specific char- 

 acter is also an adaptive character, it is reasonable to 

 suppose that it has been developed by natural selection; 

 but if, as Mr. Romanes contends, a large number of 

 specific characters are of no use, some other factor must 

 have developed them, which factor Mr. Romanes consid- 

 ers to be physiological selection. He asserts, moreover, 

 that without isolation evolution would be linear in 

 direction but never branching. The struggle for exist- 

 ence is most intense in the most populous districts, and 

 consequently the evolution of new species should be 

 most rapid in such regions, but the evolution of new 

 forms in a crowded district would be especially difficult, 

 in fact practically impossible without some form of iso- 

 lation. Physiological selection would afford this needed 

 aid. 



That so revolutionary a theory as this appeared to be 

 should not be immediately accepted by scientists was 

 not to be wondered at. To be sure it had been sug- 

 gested previously in Mr. Belt's Nicaragua and in 

 a letter to Nature by Mr. Catchpool, but Mr. Romanes 



