136 THE THEORY OF MUTATION 
was a very gradual one is to be accounted for in part, 
at least, from the fact that the methods of selection 
themselves gradually improved from year to year. 
There is no reason to doubt that a thoroughly efficient 
method of selection would have worked its full effect 
in a few generations. A similar state of things is said 
to be the case with the cereals, such as wheat and 
barley, which have been selected largely for the size 
of the grains. From his own experiments, de Vries 
has come to the conclusion that, when selection is 
really efficient, the full possible effect of this process 
is exhausted in quite a small number of generations, 
and that then the only further effect of selection is to 
keep up the standard already arrived at. 
We have seen that the theoretical conclusions of the 
biometricians are in agreement with the opinions here 
expressed, so long as selection is understood to be con- 
fined to the choosing out of parents which show a 
definite standard value of the character under con- 
sideration, this value being the same in each genera- 
tion. Under these circumstances, Professor Pearson 
concludes that in the first two or three generations 
a marked advance in the desired direction will take 
place, but that further selection (in this sense) will 
have comparatively little effect. But the believer 
in continuous evolution maintains in addition that 
selection will be followed to an indefinite extent by 
further variations in the direction of selection, since 
otherwise selection could never lead to important 
changes in organization. In the face of the strong 
contrary evidence, and of the fact that alternative 
