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THE AMERICAN NATURALIST [Vol. LII 



currence of factor mutations. When in a certain germ 

 cell of such an individual a mutation occurs which will 

 produce a detrimental or lethal effect, the offspring will 

 either die or fail to reproduce as the case may be. Hence 

 pure lines possess no lethal or highly detrimental factors ; 

 yet these biotypes may vary between wide limits in their 

 morphological and physiological characters. 



There is no necessity whatever for the simultaneous 

 appearance of mutations in order to establish new forms. 

 Only three conditions are necessary in this method of 

 evolution : the existence of species during long periods of 

 time; repeated occurrence of some factor mutations re- 

 sulting in new characters, advantageous to the species ; and 

 the transmission of these mutations from generation to 

 generation. All these conditions are known to exist. In 

 fact, the repeated occurrence of the same mutation in the 

 same locus of a particular chromosome has been observed, 

 as well as the occurrence of different factor mutations 

 producing similar somatic variations. 



Factor mutations, therefore, provide the means for 

 gradual evolution within species ; only a few, to be sure, 

 out of many factor mutations being preserved, but these 

 few being sufficient, with the frequent aid of migration or 

 isolation by geographical barriers, to build up new groups 

 which can be recognized only as distinct species. But 

 these new species, it will be understood, would have the 

 same chromosome numbers as the ones from which they 

 arose. We are, therefore, considering here only one of 

 several methods by which new species originate. Strictly 

 speaking, the only true mutations are factor mutations, 

 as they are the only known germinal variations. 



Conclusion 



Factor mutations occur in accordance with the general 

 scheme of the mutation theory as formulated by de Vries. 

 They arise suddenly, they occur in all directions, they are 

 heritable, and some of them are advantageous to the 

 species and are preserved by natural selection. When so 



