THE EVIDENCE OF EVOLUTION. 367 



inaugurated in this country. I refer to the pure university studies 

 and the work of the agricultural stations. Highly valuable is the 

 application of science to agriculture in the improvement of races. 

 Each of you knows how this artificial production of races of animals 

 and plants was one of the great sources of evidence on which Darwin 

 founded his theory. But at his time the available evidence was 

 only very scanty when we compare it with the numerous facts and 

 the improved methods which now are the result of half a century's 

 additional work. America and Europe have combined in this line, 

 and the vast amount of facts, heaped up by numerous investigators 

 and numerous well-equipped institutions, has produced quite a 

 new basis for a critical review of Darwin's theory. 



I have tried to combine all these too dispersed facts and to bring 

 them together, in order to obtain a fuller proof for the main points 

 of Darwin's conception. In one subordinate point my results have 

 been different from those of Darwin, and it is this point which I 

 have been invited by the kindness of your president to discuss 

 before you. 



Darwin's theory is commonly indicated as the theory of natural 

 selection. This theory is not the theory of descent. The idea of 

 descent with modification, which now is the basis of all evolutionary 

 science, is quite independent of the question how in the single in- 

 stances the change of one species into another has actually taken 

 place. The theory of descent remains unshaken even if our con- 

 ception concerning the mode of descent should prove to be in need 

 of revision. 



Such a revision seems now to be unavoidable, hi Darwin's time 

 little was known concerning the process of variability. It was im- 

 possible to make the necessary distinctions. His genius recognized 

 two contrasting elements; one of them he called sports, since they 

 came rarely, unexpectedly and suddenly; the other he designated as 

 individual differences, conveying thereby the notion of their presence 

 in all individuals and at all times, but in variable degrees. 



Sports are accidental changes, resulting from unknown causes. 

 In agricultural and horticultural practise they play a large part, 

 and whenever they occur in a useful direction, they are singled out 

 by breeders and become the sources of new races and new varieties. 

 Individual differences are always present, no two persons being ex- 

 actly alike. In the same way the shepherd recognizes all his sheep 

 by distinct marks, and to find two ears in a field of wheat which 

 can not be distinguished from one another by some peculiarity is a 



