524 THE AMERICAN NATURALIST [Vol. LII 



Similarly, the work of MacDougal (1907), in connec- 

 tion with the modification of Baimannia odorata, one of 

 the Patagonian primroses, may be explained. Compton, 

 as noted by Bateson (1912), using the same species, was 

 unable to obtain like results, while Humbert (1911) utiliz- 

 ing 7,500 pure line plants of Silene noctifiora, one of the 

 ''pinks" also failed to obtain so-called "mutants" simi- 

 lar to those found by MacDougal. 



The investigations of Kammerer, Woltereck, Ferro- 

 niere, etc., are of decided interest, but to those critically 

 inclined they offer no evidence giving pronounced sup- 

 port to the proposition that environmental stimuli form 

 new genetic factors. 



Thus, m turn, have the theories as to the method by 

 which evolutionary progress occurs been undermined 

 by doubt. Feeling the insufficiency of small chance varia- 

 tions, of environmental variations, and of larger germi- 

 nal variations, as a summation process, it is not to be 

 wondered that the truth-seeking pilgrim has become 

 wearied in his journey and longs for a more secure rest- 

 ing place. 



Ill 



Let us return to the problem as suggested in the open- 

 ing paragraph, namely the actual origin of heritable 

 characters, and consider somewhat more carefully as to 

 whether theories exist justified by facts, which will 

 furnish acceptable evidence. There are two well-de- 

 veloped hypotheses, the general one of DeVries and the 

 more specific one of Morgan and his associates, founded 

 on discontinuous variations, and that of Castle based on 

 continuous variations. 



Considering the views of DeVries and his followers in 

 the light of experimental investigations made during the 

 last ten years, it has become more and more evident that 

 by far the greater number, if not all, of the so-called mu- 

 tations thus obtained, were explainable on the basis of the 

 combinations of preexisting units of the germ cells. This 

 rests upon the proposition that there are present in the 



