34 Darwin, and after Darwin. 



are but two in number although but one of them 

 has been hitherto adduced. This, therefore, I will 

 take first. 



Mr. Wallace, with his customary desire to show 

 that natural selection is everywhere of itself capable 

 of causing organic evolution, seeks to minimize the 

 swamping effects of free intercrossing, and the conse- 

 quent importance of other forms of isolation. His 

 argument is as follows. 



Alluding to the researches of Mr. J. A. Allen, 

 and others, on the amount of variation presented 

 by individuals of a species in a state of nature, 

 Mr. Wallace shows that, as regards any given part of 

 the animal under consideration, there is always to 

 be found a considerable range of individual variation 

 round the average mean which goes to constitute the 

 specific character of the type. Thus, for example, 

 Mr. Allen says of American birds, "that a varia- 

 tion of from fifteen to twenty per cent, in general size, 

 and an equal degree of variation in the relative size 

 of different parts, may be ordinarily expected among 

 specimens from the same species and sex, taken at 

 the same locality, while in some cases the variation 

 is even greater than this." Now, Mr. Wallace is under 

 the impression that these facts obviate the difficulty 

 which arises from the presence of free intercrossing 

 the difficulty, that is. against the theory of natural 

 selection when natural selection is supposed to have 

 been the exclusive means of modification. For, as 

 he says, " if less size of body would be beneficial, 

 then, as half the variations in size are above and 

 half below the mean or existing standard of the 

 species, there would be ample beneficial variations " ; 



