THE MUTATION THEORY 327 



LATER INVESTIGATIONS OF MUTATIONS 



Since the publication of De Vries's classic investigations a large 

 amount of attention has been paid both by botanists and by zoologists 

 to the subject of mutations. Some of the investigators, notably B. M. 

 Davis, went far toward discrediting the whole of the exceptionally 

 careful work of De Vries by claiming that Oenothera lamarckiana is of 

 hybrid origin. It was pointed out that the form worked with is a 

 domestic type escaped from cultivation and that there is nowhere in 

 the known world any wild species comparable with it. It is supposed 

 to have been brought to Europe from America many years ago, but 

 there is no such species in America today. Davis claims that he has 

 succeeded in producing, by crossing two American wild species, a 

 hybrid form distinctly resembling Oenothera lamarckiana, and that 

 when inbred this hybrid produces offspring showing various combina- 

 tions of the two parent-species that are not unlike some of the mutants 

 observed by De Vries. Jeffreys has also pointed out that the pollen 

 grains of Oenothera lamarckiana exhibit a high percentage of sterility, 

 which he believes to be a stigma of hybridity. The general tenor of 

 this type of destructive criticism is to invalidate the whole mutation 

 theory as developed by De Vries and to reduce his mutants to the level 

 of mere Mendelian recombinations of characters once introduced from 

 two or more parental species. 



The large amount of work on the cytology of Oenothera by Gates and 

 others has, however, served to show that the mutants of De Vries are 

 more than hybrid segregates. Moreover, the beautiful work of 

 Blakeslee on the Jimson weed (Datura) and the work of many other 

 botanists, whose findings are reported by Gates in a contribution 

 quoted below, serve to indicate that the type of evolutionary behavior 

 first observed in Oenothera is by no means exceptional, but is probably 

 a common thing at least among plants and may be commoner than we 

 at present know in animals. It may be said by way of anticipation 

 of Gates' detailed account that nearly all of the mutations observed in 

 various species of plants may be definitely correlated with observable 

 changes in the chromosomes of the germ cells, involving changes in 

 number or changes in arrangement of these nuclear elements. 



While the botanists busied themselves with their type of mutations, 

 the zoologists, especially T. H. Morgan and his able collaborators, were 

 making discoveries of equal moment in connection with their studies 

 of the mechanism of Mendelian heredity in Drosophila. As has al- 



