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 zliii. p. 117. 



