THE EVIDENCE OF EVOLUTION. 391 



iiiul the \':ist aiiioiinl of facts, heiij^cd up by miiixM'ous in vest ig"a tors 

 and iiunu'roiis wcll-t'ciuippcd instilulions, has jji-odiiced (juilc a new 

 basis foi- a critical review of Dai'win's theory. 



1 have tried to conihinc all these too dispersed facts and to hrin*; 

 (hem together, in order to obtain a fullei* proof for the main i)<)ints 

 ol" Darwin's conception. In one subordinate point my resnlts have 

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

 been invited by the kindness of your [)resident to discuss before you. 



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

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

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

 science, is (juite independent of the question how in the single 

 instances the change of one species into another has actuall}^ taken 

 place. The theory of descent remains nnshaken even if our conception 

 concerning the mode of descent should [)rove to be in need of revision. 



Such a revision seeems now to be unavoidable. In Darwin's time 

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

 sil)le to niake 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 incli- 

 \ idual (liH'erences, 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 practice they play a large part, and 

 whenever they occur in a useful direction they are singled out by 

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

 Individiud dilferences are always present, no two persons being 

 exactly alike. In the same wa}^ 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 peculiarit}^ is a prop- 

 osition which everybody knows to be impossible. Many highly im- 

 proved races of forage plants and agi"icultural crops have been pro- 

 duced by intelligent breeders simply on the ground of these always 

 available dissimilarities. They can be selected and accumulated, 

 augmented and heaped up, until the new race is distinctly preferable 

 to the original strain. 



In ordinary agricultural breeding, however, it is ver}^ difficult to 

 distinguish sharply between these tAvo principles. Moreover, for 

 practical purposes, this distinction has no definite use. The practice 

 of selection is nearlj^ the same in both cases, and, besides hybridizing, 

 wnth which we are not now concerned, selection is as yet practically 

 the only means for the breeder to improve his races. Hence it came 

 that at Darwin's time there was no clear distinction between the two 

 types of variations, at least not to such an extent that a theor}^ of the 

 "origin of species could confidently rely upon it. 



