44 Darwin, and after Darwin. 



groups (by preventing fusion again into one group, 

 more or less resembling the original parent form), do 

 we not perceive at least a strong probability that 

 in the lower stages of its evolution such mutual in- 

 fertility must have acted as a segregating influence 

 between the diverging types, in a degree proportional 

 to its own development ? The importance of mutual 

 sterility as a condition to divergent evolution is not 

 denied, when this sterility is already present in an 

 absolute degree; and we have just seen that, before 

 it can have attained to this absolute degree, /'/ must 

 presumably, and as a rule, itself have been the subject 

 of a gradual development. Does it not therefore 

 become, on merely antecedent grounds, in a high 

 degree probable, that from the moment of its in- 

 ception this isolating agency must have played the 

 part of a segregating cause, in a degree propor- 

 tional to that of its completeness as a physiological 

 character ? 



Whoever answers this question in the affirmative 

 will have gone most of the way towards accepting, on 

 merely antecedent grounds, the theory of physiological 

 selection. And therefore it is that I have begun this 

 statement of the theory by introducing it upon these 

 grounds, thereby hoping to show how extremely simple 

 how almost self-evident is the theory which it will 

 now be my endeavour to substantiate. I may here 

 add that the theory was foreshadowed by Mr. Belt 

 in 1874 x , clearly enunciated in its main features by 

 Mr. Catchpool in i884 2 , and very fully thought out 

 by Mr. Gulick during a period of about fifteen years, 



1 Nicaragua, p. 307. 

 * Nature, vol. xxxL p. 4. 



