MUTATION 311 



tion in the animal and vegetable kingdom, did not 

 offer a strong stimulus to experimental research. 

 One of the greatest objections to the Darwinian the- 

 ory of descent arose from the length of time it would 

 require if all evolution was to be explained on the 

 theory of slow and all but invisible changes. This 

 difficulty is at once met and fully surmounted by the 

 hypothesis of periodical but sudden and quite notice- 

 able steps. This assumption requires only a limited 

 number of mutative periods, which might well occur 

 within the time allowed by physicists and geologists 

 for the existence of animal and vegetable life on the 

 earth." This is not, however, the only benefit which 

 the transformist ideas will derive from the applica- 

 tion of the mutation theory. One of their weak 

 points was that "the followers of the theory of de- 

 scent were induced to deny the manifest fact that 

 species are constant entities," in spite of all the ob- 

 servations pointing to the contrary. The notion of 

 a "constant" species was radically opposed to the idea 

 of a "variable" species. 



The mutation theory gives a clew to the final com- 

 bination of the two contending ideas. "A species is 

 always uniformly variable or rather is not always 

 uniformly mutable." (De Vries reserves the word 

 variability for the phenomenon of individual fluctua- 

 tion.) "Mutability is not a permanent feature but a 

 periodic phenomenon, producing at times new qualities, 



