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 physioloc^ical 

 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 " fiom the first 

 1 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 usiial 

 causes of their failure to become developed mto 

 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 iront, 



' Vol. xliii. p. 127. 



