36o READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



CRITICISMS 



[Oenothera lamarckiana has fallen under suspicion of being of 

 impure stock due to the crossing of two or more original species. The 

 basis for this suspicion is that the type found in Holland is not a truly 

 wild indigenous species, but a domestic type escaped from cultivation. 

 It seems probable that the species was imported from America a great 

 many years ago. B. M. Davis has succeeded in producing by cross- 

 ing two American wild species a hybrid form distinctly resembling 

 Oenothera lamarckiana in numerous respects, and this hybrid, like any 

 other hybrid, produces numerous combinations of the parental charac- 

 ters when inbred, and these hybrid progeny sometimes resemble the 

 mutants observed by De Vries. It is also said that the pollen grains 

 of Oenothera lamarckiana exhibit a high degree of sterilit}-, which is 

 a characteristic defect of hybrid plants. 



Whether or not, however, the Oenothera situation be taken as 

 valid evidence of the occurrence of mutations, the idea of mutations 

 and their role in evolution will stand up on quite independent grounds. 

 Numerous mutations have been observed in all sorts of animals and 

 plants. Professor T. H. Morgan and his able corps of collaborators 

 have observed in their carefully controlled breeding experiments with 

 the fruit fly Drosophila melanogaster hundreds of suddenly appearing 

 new characters which are classed by them as mutants, and whose 

 heredity has been most accurately studied. These mutants involve 

 single characters of the organism which are sometimes of a prominent 

 and readily recognizable sort and sometimes of so slight a degree as 

 to be imperceptible except to the trained eye of the expert student of 

 genetics. It also frequently occurs that two mutants are to the eye 

 practically identical, but rnay be distinguished by differences in 

 hereditary behavior. The great majority of the mutants discovered 

 in Drosophila are termed "lethals" because they involve the death 

 of the mutants possessing the variation. A further discussion of 

 Drosophila "mutants" appears in a later chapter. — Ed.] 



CAUSES OF MUTATIONS 



[Various mutations have been produced experimentally by subject- 

 ing the germ cells to radically changed environmental conditions. 

 W. L. Tower claims to have produced at least two new elementary 

 species of potato beetle by subjecting the already grown and unchange- 

 able parents to radically changed temperature and humidity conditions. 

 Although the parents could undergo no change themselves they 



