2oo Heredity and Eugenics 



any way detract from the value of DeVries' observations, 

 nor of the role which types thus arisen may play in evolu- 

 tion. It is true that the hypothetical portion of DeVries' 

 theory as regards a permutation period and so on, is in part 

 true, hi part not. There is a period of synthesis, ending 

 in a uniform stem race that may endure for a long time, and 

 subsequently this throws off from itself gametes unlike, 

 in that there are new combinations of old characters, 

 reappearance of long latent characters, and not infrequently 

 new characters. 



There is, however, this essential difference between the 

 conception of DeVries and the one that I have to offer: 

 namely, DeVries regards species as pure in the old sense 

 and arising by dichotomy, while I am convinced that prog- 

 ress will show that synthetic combinations are largely 

 responsible for the stem forms of generic groups, and that 

 from these there have arisen related species or types in 

 greater or less profusion. As far as experience goes, this 

 production of new types is in the main, if not entirely, a 

 product of the action of external forces upon the gametic 

 constitution, although when once started in such a strain 

 it seems not to cease for some time though the inciting cause 

 is removed. In Fig. 70 1 have tried to show in diagrammatic 

 fashion the essential differences of the two conceptions. 



B. The Experimental Production of Germinal Variations by 

 the Direct Action of Different Forces 



I. FORCES EXTERNAL TO THE ORGANISM 



Incident solar radiation in its various manifestations, 

 the water relations of organisms, density and composition 

 of the medium, and the nature of the food stream are the 

 common groups of forces that are apt to be modified in 



