158 THE LESSON OF EVOLUTION 



fied in the same country, and those cases in which 

 a species splits into two, or three, or more new 

 species ; and in the latter case I should think nearly 

 perfect separation would greatly aid their ' Specifi- 

 cation ' to coin a new word."** 



Dr. Wallace, however, still thinks that natural 

 selection can produce divergence ; but all the illus- 

 trations he gives in his book on Darwinism either 

 include isolation or they start with two or more 

 different species, and so miss the point, which is that 

 one species cannot give rise to two without the help 

 of isolation. Also, when discussing the influence of 

 natural selection on infertility he says that , " if two 

 forms of a species freely intercross with each other 

 and produce mongrel offspring which are quite fer- 

 tile inter se, then the further differentiation of the 

 forms into two distinct species will be retarded or 

 perhaps entirely prevented." 45 So, to help him in 

 his argument, he assumes that the mongrel offspring 

 ure not quite fertile inter se. 



But if the two forms are thus liable to oblitera- 

 tion, how could they possibly have arisen? The 

 partial sterility must have commenced before the 

 two forms could have been produced, and therefore 

 before there were any mongrels. In other words, 

 certain individuals must have been more or less 

 physiologically isolated from each other before the 

 two forms could have come into existence. This sup- 

 posed example of Dr. Wallace does not, therefore, 

 explain the difficulty, which is the origination of 



""Lif s and Letters," vol. iii., p. 160. 



4S " Darwinism," 2nd edition, p. 174. 



