60 Darwin, and after Darwin. 



1. That the theory of physiological selection is 

 opposed to the theory of natural selec' ' >n. Far from 

 this being so, it is at all events in my own opinion a 

 very important aid to it, in preventing free intercross- 

 ing on a common area, and thus allowing divergent 

 evolution to occur within that area. 



2. That, in advancing the theory of physiological 

 selection as " an additional suggestion on the origin 

 of species," I wish to represent it as being the 

 originating cause of all species. What I hold is, that 

 all species must have owed their origin to isolation, in 

 some form or other ; but that as physiological selection 

 is only one among many other forms of isolation (in- 

 cluding natural selection), and as it can only act on 

 common areas, a large number of species must have 

 been formed without its aid. 



3. That I imagine physiological varieties always 

 to arise " sporadically," or as merely individual 

 " sports " of the reproductive system. On the con- 

 trary, I expressly stated that this is not the way in 

 which I suppose the "physiological variation" to 

 arise, when giving origin to a new species ; but that 

 it arises, whenever it is effectual, as a " collective 

 variation" affecting a number of individuals simul- 

 taneously, and therefore characterizing " a whole race, 

 or strain/' 



4. That 1 suppose physiological selection always to 

 act alone. This I have never supposed. The essential 

 point is, not that the physiological isolation is un- 

 associated with other forms of isolation, but that 

 unless associated with some degree of physiological 

 isolation, no one of the other forms is capable of 

 originating species on common areas with any approach 



