Opinions on Isolation. 105 



find it adduced to illustrate the process of monotypic 

 evolution. But the fact of this analogy being unsound 

 does not affect the validity of the distinction between 

 monotypic and polytypic evolution to which it led 

 Darwin, in his later years, so clearly to express l . 



Turning next to the second point which we have to 

 notice, it is easy to show that in the earlier editions 

 of his works Darwin did not sufficiently recognize 

 the levelling effects of free intercrossing, and conse- 

 quently failed to perceive the importance of isolation 

 (in any of its forms) as a factor of organic evolution. 

 This may be most briefly shown by quoting his own 

 more matured opinion upon the subject. Thus, with 

 reference to the swamping effects of intercrossing, he 

 wrote to Mr. Wallace in 1867 as follows: 



I must have expressed myself atrociously : I meant to say 

 exactly the reverse of what you have understood. F. Jenkin 

 argued in the North British Review against single variations 

 being perpetuated, and has convinced me, though not in quite 

 so broad a manner as here put. I always thought individual 

 differences more important ; but I was blind, and thought that 

 single variations might be preserved much oftener than I now 

 see is possible or probable. I mentioned this in my former 



1 The analogy is radically unsound because unconscious selection 

 differs from methodical selection only in the degree of "separation" 

 which it effects. These two forms of selection do not necessarily differ 

 from one another in regard to the number of characters which are being 

 simultaneously diversified ; for while it may be the object of methodical 

 selection to breed for modification of a single character alone, it may, 

 on the other hand, be the result of unconscious selection to diversify an 

 originally uniform stock, as Darwin himself observes with regard to 

 horse-breeding. The real distinction between monotypic and polytypic 

 evolution is, not at all with reference to the degree of isolation (i. e. 

 amount of " separation "), but to the number of cases in which any 

 efficient degree of it occurs (i. e. whether in but a single case, or in two 

 or more cases). 



