THE EVIDENCE OF EVOLUTION. 391 



and the vast amount of facts, heaped up by numerous in vest io-a tors 

 and numerous welL-equipped institutions, has })rodu('ed (piite a new 

 basis for a eritieal review of Darwin's theory. 



I have tried to combine all these too dispersed facts and to brin<j 

 them together, in order to ol)tain a fulk'r proof for the main points 

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

 been different from those of Dai-win. and it is this point which 1 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 

 instances the change of on.e species into another has actually taken 

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

 concerning the mode of descent should prove to be in need of revision. 



Such a revision seeems now to be unavoidal)le. In Darwin's time 

 little was known concerning the process of ^'ariability. It was impos- 

 sil)le to u'lake the necessary distinctions, llis genius recognized two 

 contrasting elements — one of them he called sports, since they came 

 rarely, unexpectedly, and suddeidy; the other he designated as indi- 

 vidual diiferences, conveying thereby the notion of their jjresence in 

 all individuals and at all times, but in varial)le degrees. 



kSports ai-e accidental changes, resulting from unknown causes. 

 In agricultural and horticultural practice they play a large part, and 

 whentM'er they occur in a useful direction they are singl(Ml out by 

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

 Individual iliU'erences are always present, no two persons being 

 exactly 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 l)y some peculiarity is a prop- 

 osition which everybody knows to be imjjossible. Many hiuidy im- 

 proved races of forage plants and agricultural cro])s have been pro- 

 duced by intelligent breeders simply on the ground of these always 

 available dissimilarities. They can be selected and accumidated, 

 augmented and heaped up, until the new race is distiiu-th' ))iefei'able 

 to the original strain. 



In ordinary agricultural breeding, however, it is very dilKcult to 

 distinguish sharply between these two princii)les. Moreover, for 

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

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

 with which we are not now concerned, selection is as j^et 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 ext(>nt that a theory of the 

 origin of species could confidently rely upon it. 



