158 Darwin, and after Darwin. 



previously and frequently stated, I have so strong a belief 

 in the segregating power of physiological selection, or 

 selective fertility, that I do not think it is necessary for 

 this principle to be always associated with some other form 

 of homogamy. From the first, indeed, I have laid great 

 stress (as. also, has Mr. Gulick) on the re-enforcing influence 

 which association with any other form of homogamy must 

 exercise upon the physiological form, and vice versa ; but 

 I have also said that, in my opinion, the physiological form 

 may in many cases be able to act entirely alone, or without 

 assistance derived from any other source. The question 

 here is, as we have already so fully seen, a question of but 

 secondary importance ; since, whether or not the physio- 

 logical form of homogamy ever acts alone, even Mr. Wallace 

 now allowS; or rather argues, that it acts in combination — 

 and this so habitually, as well as with so much effect, that it 

 constitutes a usual condition to the origination of species. 

 Nevertheless, although the only relevancy of his numerical 

 computation of chances — whereby he thinks that he over- 

 turns my theory in toto — is such relevancy as it bears to this 

 question of secondary importance, I have thought it desirable 

 to refer the question, together with Mr. Wallace's views upon 

 it, to the consideration of a trained mathematician. 



As this " subordinate question " depends entirely on 

 numerical computations involving the doctrine of chances, 

 I should first of all like to remark, that in reference to 

 biological problems of the kind now before us, I do not 

 myself attach much importance to a merely mathematical 

 analysis. The conditions which such problems involve are 

 so varied and complex, that it is impossible to be sure about 

 the validity of the data upon which a mathematical analysis is 



that utility is not the only possible means of securing effective isolation) 

 I here neglect it, and take the wider ground marked out above. It is 

 needless to say that this is gi^'ing Mr. Wallace every possible advantage, 

 by not holding him to his still narrower ground. 



