THE AMERICAN NATURALIST 



[Vol. LI 



demonstrative evidence that we rely when we champion the doctrine of 

 grounds." 



Even since 1909 a good deal of water has flowed under 

 all our bridges, and particularly under the evolutionary 

 ones. Among other changes in viewpoint there is evident 

 a marked disinclination in science nowadays to regard as 

 ''scientific truth" anything which is not based upon 

 demonstrative evidence. But it is also a fact, perhaps at 

 first thought to be regarded as curious, in view of the 

 opinion of Weismann which has been quoted, that there 

 are here with us to-day those who assert, with great zeal 

 and pertinacity, that in selection is to be found the chief 

 cause of evolutionary change. These things being so, it 

 has seemed that possibly it might be profitable to spend 

 a little time upon the selection problem, trying to deter- 

 mine whether the case is any better now than Weismann 

 conceived it to be seven years ago, from the viewpoint of 

 tangible objective evidence. It is to be hoped that it is, 

 for among working geneticists just now any theory 

 which has to depend for its sole support upon its ^'inter- 

 pretative value "3 is sure to receive scant attention. 



So then what I shall try to do is to review briefly some 

 of the real evidence about the selection problem which 

 has been accumulating since biologists turned definitely 

 to the experimental study of evolution, and definitely 

 away from the glorious, but on the whole unproductive, 

 attempt to solve its problems by a priori reasoning. 

 From such a review it is to be hoped that we may get 

 some light as to the directions in which further research 

 and new evidence are most urgently needed. 



I. Natural Selection 

 In considering the whole selection problem we may 

 well begin with an examination of the theory of natural 

 selection. The mere fact of natural selection, in the sense 

 solely and strictly of a process leading to the elimination 

 of some individuals and the survival of others, is noi 



