38 Darwin, and after Darwin. 



gamy other than natural selection, it must effectually 

 inhibit any segregation of specific types, or divergence 

 of character. 



Hence it is that, while no Darwinian has ever 

 questioned the power of unaided selection to cause 

 improvement of character in successive generations, in 

 common now with not a few other Darwinians I have 

 emphatically denied so much as the abstract possi- 

 bility of selection alone causing a divergence of char- 

 acter in two or more simultaneous lines of change. 



And, although these opposite views cannot be 

 reconciled, I am under the impression that they do 

 admit of being explained For I take them to 

 indicate a continued failure to perceive the all-im- 

 portant distinction between evolution as monotypic 

 and polytypic. Unless one has fully grasped this 

 distinction, and constantly holds it in mind, he is 

 not in a position to understand the " difficulty " in 

 question ; nor can he avoid playing fast and loose 

 with natural selection as possibly the sole cause of 

 evolution, and as necessarily requiring the co-operation 

 of some other cause. But if he once clearly perceives 

 that "evolution" is a logical genus, of which the mono- 

 typic and the polytypic forms are species, he will 

 immediately escape from his confusion, and find that 

 while the monotypic form may be caused by natural 

 selection alone the polytypic form can never be 

 so caused. 



The second difficulty which I have to mention as at 

 first sight attaching to the views of Mr. Gulick and 

 myself on the subject of Isolation is, that in an iso- 

 lated section of a species Mr. Francis Galton's law of 



