September 23, 1904.] 



SCIENCE. 



397 



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 one 

 species into another has actually taken 

 place. The theory of descent remains un- 

 shaken even if our conception concerning 

 the mode of descent should prove to be in 

 need of revision. 



Such a revision seems now to be unavoid- 

 able. In Darwin's time little was known 

 concerning the process of variability. It 

 was impossible to make the necessary dis- 

 tinctions. His genius recognized two con- 

 trasting 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 indi- 

 viduals 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 proposi- 

 tion which everybody knows to be impos- 

 sible. Many highly improved races of for- 

 age plants and agricultural crops have been 

 produced by intelligent breeders simply on 

 the ground of these always available dis- 

 similarities. They can be selected and ac- 

 cumulated, augmented and heaped up, 



until the new race is distinctly preferable 

 to the original strain. 



In ordinary agricultural breeding, how- 

 ever, it is very difficult to distinguish 

 sharply between these two principles. 

 Moreover, for practical purposes, this dis- 

 tinction has no definite use. The practise 

 of selection is nearly the same in both cases,. 

 and, besides hybridizing, with which we are 

 not now concerned, selection is as yet prac- 

 tically the only means for the breeder tO' 

 improve his races. Hence it came that at 

 Darwin's time there was no clear distinc- 

 tion between the two types of variations, 

 at least not to such an extent that a theory 

 of the origin of species could confidently 

 rely upon it. 



Quetelet's celebrated law of variability 

 was published only some years after the 

 appearance of Darwin's 'Origin of Spe- 

 cies.' Variability seemed until then to be 

 free from laws, and nearly everything could 

 be ascribed to it or explained by it. But 

 the renowned Belgian scientist showed that 

 it obeys laws exactly in the same way as 

 the remainder of the phenomena of nature. 

 The law which rules it is the law of prob- 

 ability, and according to this law the occur- 

 rence of variations, their frequency and 

 their degree of deviation can be calculated 

 and predicted with the same certainty as 

 the chance of death, of murders, of fires 

 and of all those broad phenomena with 

 which the science of sociology and the prac- 

 tise of insurance are concerned. 



The calculations of probable variations 

 based on this most important law did not, 

 however, respond to the demands of evo- 

 lution. Specific characters are usually 

 sharply defined against one another. They 

 are new and separate units more often than 

 different degrees of the same qualities. 

 Only with such, however, Quetelet's law is 

 concerned. It explains the degrees, but 

 not the origin, of new peculiarities. More- 

 over, the degrees of deviation are subject 



