322 Plant Life and Evolution 



tion that the sexual cells contain certain structures, 

 " pangenes," which are the transmitters of heredi- 

 tary characters. He assumes also that there are 

 certain periods in the history of the species, when for 

 some reason a state of great instability and great 

 variability prevails, and that at such periods muta- 

 tions occur. A mutation is to be explained by the 

 development of an additional one of the " heredity 

 bearers " in the germ cells of the mutant, or by 

 the destruction of one of these ; while in mere varie- 

 ties, the number of these hereditary units is the 

 same as in the parent species, but one or more of 

 them may remain latent. While such an explana- 

 tion is logical enough, it must be confessed that it is 

 quite incapable of proof, and may very well be re- 

 garded with some skepticism. 



De Vries' experiments on (Enothera Lamarckiana 

 have been repeated in America, especially under the 

 direction of MacDougal in New York, and the re- 

 sults are entirely in accord with those obtained by 

 De Vries. 



Darwin regards the accumulation of slight fluc- 

 tuating variations as the basis upon which natural 

 selection works, while De Vries contends that these 

 fluctuating variations are of no value in evolution, 

 and that mutations alone are important; but he 

 agrees with Darwin that the variation, once estab- 

 lished, must depend upon natural selection for its 

 maintenance. Whether the mutation theory will 

 prove the all-sufficient explanation of the origin of 



