6 Outline of Genetics 



adequate can be considered more critically a little later 

 in the light of the more recent knowledge of inheritance. 



The third great explanation of evolution was offered 

 by De Vries in 1900. This author was the first to base 

 his conclusions on the results of his own experimental 

 breeding, rather than merely on the extensive observa- 

 tion of plants and animals in nature. Judging from the 

 behavior shown by Oenothera Lamar ckiana (American 

 evening primrose) during the course of the ten or more 

 generations that it grew in his garden, De Vries con- 

 cluded that the real basis of evolution lay in the phenom- 

 enon of mutation. In addition to its ''normal" pro- 

 geny, 0. Lamarckiana produced in small numbers cer- 

 tain distinctly new types, the mutants. The type of 

 variation involved in mutation was distinctly different 

 from the Darwinian, being qualitative, discontinuous, 

 and constant. It was readily seen that the mutants 

 involved qualitative changes from the parent, inasmuch 

 as entirely new characters were shown, rather than merely 

 the quantitatively greater or lesser development of cer- 

 tain of the parental characteristics. It was equally 

 plain that mutation was discontinuous, the direction 

 and nature of mutations being entirely unrelated to any 

 of the mutations that had taken place in the past. And 

 finally, the mutants were strikingly constant, breeding 

 true to their own characteristics rather than reverting 

 in later generations to the original parental type. These 

 mutational changes that De Vries studied intensively 

 in Oenothera were later identified in other species as well. 



The part that mutation may play in evolution is 

 suggested by a consideration of the characteristics of the 

 mutants. Probably the majority of the mutants differ 



