H4 THE THEORY OF MUTATION 



result of Pearson's calculations, described in the 

 preceding chapter, seems to indicate that the selection 

 of a certain value of a particular character for many 

 generations will never lead to the formation of a race 

 in which the mean value of the character is as high 

 as the selected value. But, says the selectionist, it 

 will happen in Nature that as the standard of the 

 race is raised by selection, the value selected will be 

 still further raised, and so on, and in this way an 

 indefinite amount of improvement is rendered possible. 

 If Johannsen's conclusions are well founded, this is 

 clearly not the case ; on the contrary, there is a 

 perfectly definite limit to the effect which selection 

 can produce. 



The question whether or not a gradual method of 

 evolution is possible has not yet been absolutely 

 decided for any single species or character, but it cer- 

 tainly seems that now for the first time the possibility 

 of a definite decision is within sight. At the same 

 time it is impossible to prove a universal negative If 

 we look at the other side of the problem we shall find 

 that the evidence in favour of an alternative process 

 has multiplied even faster than the evidence against 

 the continuous accumulation of minute differences ; 

 and the present tendency is certainly to look for other 

 sources of specific distinctness than that which is 

 offered by the natural selection of continuous varia- 

 tions. 



Even before the new evidence which we have briefly 

 outlined was available, Herbert Spencer found the 

 difficulties in the way of accepting the purely 



