EFFECT OF SELECTION 135 



variations of a definite or discontinuous type the 

 fact that differences of the kind which are constantly 

 used to distinguish natural species can and do arise 

 in Nature at a single step, so that it is not necessary 

 for such differences to be built up gradually by the 

 action of natural selection. 



De Vries, in his ' Mutations Theorie,' goes further 

 than this, and attacks the position held by those who 

 accept the doctrine that natural selection of individual 

 differences can ever lead to definite and permanent, or 

 specific, distinctions. Indeed, one of the chief contri- 

 butions of this author to the species controversy is to 

 point out that the belief that artificial selection acts 

 in this way upon domestic plants is based upon a mis- 

 apprehension. De Vries himself has carried out a 

 number of experiments in selection, and he comes to 

 the conclusion that selection of ordinary individual 

 differences has no permanent effect at all.* The actual 

 effect of this kind of selection is well illustrated by the 

 results of the processes employed in the sugar-beet 

 industry, in which elaborate care is taken to select 

 those roots which contain the highest percentage of 

 sugar for the purpose of propagation. This process 

 was followed at first by a rapid improvement, but the 

 rate at which the percentage of sugar increased soon 

 fell off, until at the present day all that selection can 

 effect is to keep up the standard of excellence already 

 attained. Moreover, that this process of improvement 



* Compare, however, Johansson's more recent conclusions 

 see p. 1 1 8). 



