DOCTRINE OF EVOLUTION AND RELATED IDEAS 487 



form them. This new compound is different from any 

 ever formed before. 



2. Preservation, repetition, and accumulation of variations from 

 generation to generation. Only this can insure evolution. There 

 are two problems: 



a. Preservation of fluctuations that come to the body. If 

 there is an early separation of germ cells and body cells and they 

 lead a parallel course of development, it becomes a problem 

 whether a special quality that is gained by the body cells in 

 their life can be imparted to their cousins among the germ cells. 

 Unless they can do this it is clear that these changes cannot be 

 transmitted to the next generation, since the body cells do not 

 themselves pass over. A body may have acquired a very strik- 

 ing quality without any part of this quality being found in the 

 germ cells. 



b. Preservation of changes that come to the substance of the 

 germ cells. Clearly, since the germ cells make the next genera- 

 tion, changes in these cells may very well influence both the germ 

 and body cells of the next generation. Biologists are coming 

 more and more to feel that this is the really great field of heredi- 

 tary influence, and that evolutionary studies must concern 

 themselves increasingly with the history of these cells and the 

 kinds of influences that can change them. 



3 . The Guidance of Evolution . This might take place in either, 

 or both, of two ways. 



a. Variation might itself be guided from within, so that 

 chiefly suitable or appropriate variations shall occur. 



b. Variations may have any range, and the environment 

 through its life and death pressure on the organisms may elimi- 

 nate some and select others on the basis of fitness. The result 

 would be to make evolution follow the demand of the environ- 

 ment independently of the range of the variations. 



490. Variability and Variation. This furnishes the materials 

 of evolution. All organisms vary. Variation suggests two or 

 three things: (i) An animal may differ from day to day as the 

 result either of its own activities or of the action of the environ- 

 ment upon it; (2) it may differ, at any stage or condition of its 



