i6S Heredity and Eiitioiics 



the external conditions are tlie variable in the complex, 

 and elimination or understanding of the most obvious \'ari- 

 able is the first step in the study of gametic constitution 

 and modification. Concretely, then, what is the role of 

 external factors in the production of germinal variations ? 

 Satisfactory evidence as to the role of external factors can 

 be obtained only through careful experiments. These may 

 be either experiments under laboratory conditions, or ex- 

 periments in nature, and, if possible, both should be carried 

 on at the same time upon the same materials. 



A. The Idea of Sudden Transmutation in the Germinal 

 Material 



Succeeding Darwin, there arose a group of followers — the 

 neo-Darwinians. Possessing all the attributes of followers, 

 unable to grasp the breadth of view of their master, and see- 

 ing but a particular phase of his general teachings, they 

 endeavored to raise that to undue prominence and make it 

 a universal motive force in the evolution of organisms. 



The neo-Darwinians in the last quarter of the nineteenth 

 century, under the leadership of such men as Weismann, 

 created what Eimer has termed the "principle of omnipo- 

 tent natural selection." It was attempted to establish pur- 

 poseful selection as the sole efiicient cause oi variation and 

 e\"olution in organisms, and in the effort, Weismann went 

 so far as to place the selective process, not in the outside 

 world, but in a microcosm within the germ plasm, making 

 it in every wa}- incapable of investigation, imp<3ssible of 

 obser\'ation, and all-inclusive. 



P\wv today attach much importance to Weismann's 

 "germinal selection," and the Weismannian theorv remains 

 in biological history one of those curious ideas comparable 



