240 SECRETS OF ANIMAL LIFE 



and less to fluctuations. (As for individually 

 acquired modifications, imposed on the body from 

 without, instead of emerging from the changeful 

 germ -plasm within, they do not seem to be in 

 themselves of much direct racial importance, for 

 there is no cogent evidence of their transmissibility.) 

 Darwin knew, of course, of some of the transilient 

 or saltatory variations, which are now called muta- 

 tions, but he deliberately passed them by and laid 

 emphasis on the selection of fluctuations. His 

 strongest reason for so doing was his conviction 

 that the sudden " single variations " or sports 

 would be readily swamped or leveled down by 

 inter-crossing. It is now known, however, that one 

 of the characteristics of mutations is their capa- 

 bility of complete inheritance in a varying percent- 

 age of the progeny. 



To the Dutch botanist De Vries especial credit is 

 due for his recognition of the evolutionary im- 

 portance of mutations and for his study of their 

 behavior in inheritance. It is an often-told story 

 how he found, in 1886, in a potato-garden near 

 Hilversum, in Holland, a race of the Evening Prim- 

 rose, CEnothera laniarckiana, in which the mood was 

 all mutation. In spite of Galton's insistence on the 

 reality of transilient variations and Bateson's mar- 

 shaling of instances of discontinuity, the tendency 

 had grown strong to dogmatize about the con- 

 tinuity of organic change, just as previously about 

 the fixity of species. " Natura non facit saltus," 

 they said; but De Vries discerned Natura saltatrix 



