1S90 PHYSIOLOGICAL SELECTION 173 



acting on individuals) ; and whether natural selection 

 could in any case act on a type is a question which 

 your father has told me he could never quite make 

 up his mind about, except in the case of social 

 hymenoptera and moral sense of man. 



You will see what I mean by ' secondary varia- 

 tions ' by looking at page 366 of my paper. ' It is 

 merely a short-hand expression for all other specific 

 diSerences save the sexual difference of sterility. My 

 view is that these secondary differences are always 

 sure to arise sooner or later in some direction or another 

 wherever a portion of a species is separated from the 

 rest, whether by geographical or physiological isolation, 

 which, iadeed, as regards the former, is no more than 

 you (following Weismann, &c.) acknowledge. Now, to 

 me it seems obvious that Weismann's ' variations ' 

 {i.e. shght changes in the form of shells) cannot 

 possibly be themselves my ' physiological sports,' 

 although they may very well be the consequences of 

 such a sport leading to physiological isolation, and so 

 to independent variation in two or three directions 

 simultaneously, till afterwards blended by inter- 

 crossing. And my reason for thinking this is that 

 ' Weismann's variations ' always arose in crops at 

 enormously long intervals of time. On the mere 

 doctrine of chances it therefore becomes impossible 

 to suppose that each of these variations was due to 

 a separate physiological sport, although it is easy to 

 see how each crop of them might have been so. For, 

 if not, why should they always have arisen in crops, 

 each member of which was demonstrably fertile with 

 the other members of that crop, while no less 



