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, Johanssen’s more recent conclusions 
see p. 118). 
