136 Darwin, and after Darwin. 



change to be, "as a general rule," the primordial 

 change. At the same time, I have always been 

 careful to insist that this opinion had nothing to do 

 with "the essence of physiological selection"; seeing 

 that "it was of no consequence" to the theory in 

 what proportional number of cases the cross-sterility 

 had begun per se, had been superinduced by morpho- 

 logical changes, or only enabled to survive by 

 happening to coincide with any other form of 

 homogamy. In short, ' ; the essence of physiological 

 selection" consists in all cases of the diversifying effect 

 of cross-infertility, whensoever and howsoever it may 

 happen in particular cases to have been caused. 



Thus I emphatically reaffirm that "from the first 

 I have always maintained that it makes no essen- 

 tial difference to the theory in what proportional 

 number of cases they [the physiological variations] 

 have arisen 'alone in an otherwise undifferentiated 

 species ' " ; therefore, " even if I am wrong in sup- 

 posing that physiological selection can ever act 

 alone, the principle of physiological selection, as I 

 have stated it, is not thereby affected. And this 

 principle is, as Mr. Wallace has re-stated it, 'that 

 some amount of infertility characterizes the distinct 

 varieties which are in process of differentiation into 

 species' infertility whose absence, 'to obviate the 

 effects of intercrossing, may be one of the usual 

 causes of their failure to become developed into 

 distinct species."' 



These last sentences are quoted from the corre- 

 spondence in Nature^, and to them Mr. Wallace replied 

 by saying, " if this is not an absolute change of front, 

 1 Vol. xliii. p. i a;. 



