ORIGIN OF THE SPECIFIC TYPE 327 



De Vries regards it as a mistake to believe that artificial selection, 

 persevered in for a long time, will succeed in producing a breed which 

 will — as he expresses it— be independent of further selection and will 

 maintain itself in purity. Experience cannot decide this, as we have 

 not command over the unlimited time necessary for selection, but 

 theoretically it is quite intelligible that a variation which had arisen 

 through selection would be more apt to breed true the longer selection 

 was practised, and there is nothing to prevent it becoming ultimately 

 quite as constant as a natural species. For, at the beginning of 

 breeding, we must assume that the variation is contained in only 

 a small number of ids; as the number of generations mounts up, more 

 and more numerous ids with this variation will go to make up the 

 germ-plasm, and the more the breed-ids preponderate the less likeli- 

 hood will there be that a reversion to the parent-form will be brought 

 about by the chances of reducing-division and amphimixis. That 

 most if not all breeds of pigeon still contain ids of the ancestral form 

 in the germ-plasm, although probably only a small number of them, 

 we see from the occasional reversion to the rock-dove which occurs 

 when species are repeatedly crossed, but that ancestral ids may also 

 be contained in the germ-plasm of long- established natural species is 

 shown b}^ the occurrence of zebra-striping in horse-hybrids. We can 

 understand why these ancestral ids should not have been removed 

 long ago from the germ-plasm by natural selection, since they are not 

 injurious and may remain, so to speak, undetected. It is only when 

 they have an injurious effect by endangering the purity of the 

 new species-type that they can and must be eliminated by natural 

 selection, and this does not cease to operate, as the human breeder does, 

 but continues without pause or break. 



I therefore regard it as a mistake on the part of De Vries to 

 exclude fluctuating variation from a share in the transformation of 

 organisms. Indeed, I believe that it plays the largest part, because 

 adaptations cannot arise from mutations, or can only do so ex- 

 ceptionally, and because whole families, orders, and even classes are 

 based on adaptations, especially as regards their chief characters. 

 I need only recall the various families of parasitic Crustaceans, the 

 Cetaceans, the birds, and the bats. None of these groups can have 

 arisen through saltatory, perhaj)s even retrogressive, ' mutation ' ; they 

 can only have arisen through variation in a definite direction, which 

 we can think of only as due to the selection of the fluctuations of the 

 determinants of the germ-plasm which are continually presenting 

 themselves. 



The difference between ' fluctuating ' variability and ' mutation ' 



