320 THE THEORIES OF EVOLUTION 



elude the useful ones, and if this is repeated a number 

 of times, cumulation is possible ; if not, there is simply 

 no progression, and the type remains stable through 

 the ages. Natural selection is continually acting as a 

 sieve, throwing out the useless changes and retaining 

 the real improvements." 3 



While De Vries opposes his theory of the origin of 

 species by sudden variations to the theory postulating 

 the accumulation of slow variations, he nevertheless 

 attributes an important role to natural selection. He 

 considers selection, however, as being operative be- 

 tween species and not between individuals of the same 

 species. 



"Intraspeciflc selection produces the local races, the 

 marks of which disappear as soon as the special ex- 

 ternal conditions cease to act. It is of very subordi- 

 nate importance. The experience of breeders proves 

 that selection according to a constant standard soon 

 reaches a limit which it is not capable of transgress- 

 ing. In order to attain further improvements the 

 method of selection itself must be improved. Nature, 

 as far as we know, changes her standards from time 

 to time only in consequence of the migrations of the 

 species, or of local changes of climate." 



Selection between species is much more important 

 whether it is natural or artificial. It constitutes the 

 sieve through which certain species only can pass and 



e Pp. 558-574. 



