68 Darwin, and after Darwin. 



constitute any unusual reason for its modification by 

 survival of the fittest. On the other hand, as just 

 observed, I do very plainly see why such a reason 

 is furnished for the modifying influence of physio- 

 logical selection. 



Let us next turn to another of Darwin's general 

 rules with reference to distribution. He took a great 

 deal of trouble to collect evidence of the two following 

 facts, namely, (i) that "species of the larger genera 

 in each country vary more frequently than the species 

 of the smaller genera " ; and (2) that " many of the 

 species included within the larger genera resemble 

 varieties in being very closely, but unequally, related 

 to each other, and in having restricted ranges^." 

 By larger genera he means genera containing many 

 species ; and he accounts for these general facts by 

 the principle, " that where many species of a genus 

 have been formed, on an average many are still 

 forming." But how forming ? If we say by natural 

 selection alone, we should expect to find the multi- 

 tudinous species differing from one another in respect 

 of features presenting well-marked adaptive meanings ; 

 yet this is precisely what we do not find. For 

 Darwin's argument here is that " in large genera the 

 amount of difference between the species is often 

 exceedingly small, so that in this respect the species 

 of the larger genera resemble varieties more than do 

 the species of the smaller genera." Therefore the 

 argument, while undoubtedly a very forcible one in 

 favour of the fact of evolution, appears to me scarcely 

 consistent with the view of this evolution being due 

 solely to natural selection. On the other hand, the 



' Origin of Species, pp. 44, 45. 



