MUTATION THEORY OF DE VRIES 91 



features of animals and plants do not exist between the elements 

 themselves, any more than they do between the elements of the 

 chemist." 



It is interesting to notice that whether we consider Weismann's 

 theory of the determinants composing the germ-plasm, or the 

 Mendelian theory of the segregation of characters in the germ- 

 cells, or De Vries's Mutation Theory, we are led to the theoretical 

 conception of elementary units. And again, we find the late 

 Professor Weldon referring to Galton's Law in the following 

 terms : " The Galtonian theory postulated the presence of 

 ' elements ' in the germ-cells of one generation, which are of two 

 kinds — viz. active or dominant elements and groups of elements 

 which determine the heritable characters of the progeny or 

 second generation, and latent or recessive elements which passed 

 through the bodies of one or more generations without appearing 

 to affect them " {Lancet, March 25th, 1905, p. 810). 



The Case of the EYening Primrose. — In 1886, De Vries began 

 hunting about around Amsterdam for a plant which would show 

 hints of being in what we may caU a changeful mood. He tried 

 over a hundred species, bringing them under cultivation, but 

 almost all were disappointingly conservative. It seemed as if 

 most of the species around Amsterdam were in a non-mutable 

 state. It is possible, as Weismann suggested in one of his first 

 evolutionary essays (1872), that in the life of species periods of 

 constancy alternate with periods of changefulness. The human 

 historian has often made a similar remark. 



In the course of his wanderings around Amsterdam, De Vries 

 came across a deserted potato-field at Hilversum — a field of 

 treasure for him. For there he found his long-looked-for mutable 

 plant, an evening primrose ((Enothera lamarckiana). Like its 

 nearest relatives, (Enothera biennis and (Enothera muricata, which 

 it excels in size and beauty of flowers, it probably came from 

 America, where it is a native. It had probably "escaped" at 

 Hilversum about 1875, and in the following ten years it had 



