6 



THE AMERICAN NATURALIST [Vol.XLVIII 



admission of disbelief in the inheritance of ordinary 

 adaptive changes. The latter conception was Weismann- 

 ian in that all inherited variations were held to be changes 

 in the germ cells. It was not necessary to suppose it im- 

 possible for the environment to produce such changes and 

 therefore to have been of no value during the course of 

 evolution, but merely to suppose that during the compara- 

 tively short period of experimental investigations no gam- 

 etic variations have occurred traceable to such a cause. 

 For his first conclusion to be justified, it was assumed that 

 the changes which every biologist knows do follow the 

 continuous selection of extremes under certain conditions 

 are to be interpreted entirely by the segregation and re- 

 combination of hypothetical gametic factors which are 

 constant in their reactions under identical conditions. 



Numerous investigators working on "pure lines" with 

 different material corroborated Johannsen's conclusions, 

 and, as it was seen to be possible to interpret in the same 

 manner changes made by selection in experiments where 

 self -fertilized lines were not used, such as those of the 

 Vilmorins and others on sugar beets and those of the 

 Illinois Agricultural Experiment Station on maize, many 

 biologists accepted them and considered them a great ad- 

 vance over former conceptions of the mechanism of 

 heredity. On the other hand, there were those who main- 

 tained a skeptical attitude, the chief criticism directed 

 against the conception being that all progress due to 

 selection must have a limit, which in many of these ex- 

 periments had already been reached, and that even if re- 

 sults were being obtained action might be too slow to be 

 detected. 



The Material 

 These criticisms were reasonable when applied to cer- 

 tain specific cases, and in 1908 the experiments reported 

 in this paper were designed with the hope of testing their 

 validity, using the species ordinarily grown for commer- 

 cial tobacco, Nicotiana tabacum, as the material. This 

 plant satisfies the conditions which are requisite for 



