120 Darwin, and after Darwin. 



polytypic evolution (whether with or without natural 

 selection) depends upon the absence of it ; sixth, that, 

 having regard to the process of evolution throughout 

 all taxonomic divisions of organic nature, we must 

 deem the physiological form of isolation as the most 

 important, with the exception only of natural 

 selection. 



The only difference between Mr. Gulick's essays 

 and my own is, that, on the one hand, he has 

 analyzed much more fully than I have the various 

 forms of isolation ; while, on the other hand, I have 

 considered much more fully than he has the particular 

 form of physiological isolation which so frequently 

 obtains between allied species. This particular form 

 of physiological isolation I have called " physiological 

 selection," and claim for it so large a share in the 

 differentiation of specific types as to find in it a 

 satisfactory explanation of the contrast between 

 natural species and artificial varieties in respect of 

 cross-infertility. 



Mr. Wallace, in his Darwinism, has done good 

 service by enabling all other naturalists clearly to 

 perceive how natural selection alone produces mono^ 

 typic evolution namely, through the free intercross- 

 ing of all individuals which have not been eliminated by 

 the isolating process of natural selection itself. For 

 he very lucidly shows how the law of averages must 

 always ensure that in respect of any given specific 

 character, half the individuals living at the same time 

 and place will present the character above, and half 

 below its mean in the population as a whole. Con- 

 sequently, if it should ever be of advantage to a species 



