212 THE PROBLEMS OF EVOLUTION 



expression it governs, it is obvious that the condi- 

 tion governing its development and maintenance 

 is available throughout the body. The significant 

 aspect of this hypothesis is that the result is defi- 

 nitely associated with some condition of individual 

 life. Whatever change may occur is only a reduc- 

 tion or enhancement of something already present, 

 but it takes place according to the demands of the 

 environment. In this the hypothesis is more 

 satisfying and more in harmony with the known 

 facts of evolution than the equally hypothetical 

 idea that evolution has proceeded entirely from 

 the random influences of rays upon the structure 

 of genes. It offers equal possibility that a degree 

 of development, once attained, may find a new 

 importance to the individual under new environ- 

 mental contacts without depending entirely upon 

 this preadaptive value for its eflScacy. Neither 

 idea is, at present, susceptible to proof. 



This idea of the development of the chromosomes 

 harmonizes with the known facts of embryology 

 and with the emergent interpretation of evolution. 

 We cannot avoid the conviction that the chromo- 

 somes are always the controlling center of the cell, 

 yet the same chromosome complex accomplishes a 

 succession of varying results during the course of 

 ontogeny. Each generation of chromosomes arises 

 from a previous generation which is apparently 

 similar, yet each brings about new stages of growth 



