DE VRIES'S THEORY OF MUTATIONS. 211 



In order that any great amount of modifications should be effected by a 

 species, a variety when once formed must again perhaps after a long interval 

 of time, vary or present individual differences of the same favorable nature as 

 before; and then must again be preserved and so onward step by step. 



Those lines contain an abstract of de Vries's mutation theory. And 



then further, on page 72 : 



It should not, however, be overlooked that certain rather strongly marked 

 variations, which no one would rank as mere individual differences, frequently 

 occur owing to a similar organization being similarly acted on. . . . There 

 can also be little doubt that the tendency to vary in the same manner has often 

 been so strong that all the individuals of the same species have been similarly 

 modified without the aid of any form of selection. Or only a third, fifth or 

 tenth part of the individuals may have been thus affected, of which fact several 

 instances could be given. For cases of this kind, if the variations were of a 

 beneficial nature, the original form would soon be supplanted by the modified 

 form, through the survival of the fittest. 



Again, when Darwin denies having said that time alone plays a part 



in the process of modification which changes one species into another, 



he writes (p. 82, I. c.) : 



Lapse of time is only so far important, and its importance in this respect 

 is great, that it gives a better chance of beneficial variations arising and of their 

 being selected, accumulated and fixed. 



In the fragments which I have quoted Darwin appears to have had 

 before his mind mutation, not fluctuating variation. And I must 

 insist on the fact that de Vries makes a point of showing that Darwin 

 was decidedly inclined to accept the process of mutation. De Vries 

 quotes (p. 25) from Darwin's 'Life and Letters' (p. 87, Vol. II.) and 

 from the 'Origin of Species,' e. g., the following words: 



The formation of a . . . species I look at as almost wholly due to the 

 selection of what may be incorrectly called chance variations . . . unless 

 such occur, natural selection can do nothing, and he adds: It is obvious that 

 Darwin has attributed a great and often a preponderating, perhaps even an 

 exclusive significance to the single variations. . . . 



The chance variations were not for Darwin the extreme cases of fluc- 

 tuating variability, that can be everywhere observed; they were fortu- 

 itous phenomena. For these natural selection is always on the look- 

 out, or as Darwin has it, metaphorically, 'He catches hold of them, 

 whenever and wherever opportunity offers.' Darwin must have been 

 inclined to think that these variations, these mutations, arise in accord- 

 ance with certain laws which are entirely unknown to us. In conse- 

 quence of the operation of these laws at least a certain number of 

 favorable modifications must inevitably arise after a given lapse of 

 time. Hence the gradual evolution which most living organisms have 

 undergone in the course of ages. 



Darwin also undoubtedly suspected the existence of a certain 

 periodicity. 'Nascent species are more plastic,' he says; and he 



