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 



