440 EVOLUTION AND GENETICS 



kind throughout the body may, through the coordination of the 

 individual organism, exert a controlling influence over the part 

 which is capable of expressing them. If such is the case, the 

 spermatogonia and oogonia are no less subject to the coordinating 

 influences. Any increase or decrease in the functional capacity 

 of a gene would therefore be characteristic of the genes in these 

 germ cells as well as of those in the somatic cells expressing the 

 character. The transmission of the change to succeeding genera- 

 tions would be assured. As time went on the effect of continued use 

 might readily bring alwut a development of the genetic function 

 and an accessory development of associated parts of the internal 

 environment so great that the original stimulus would be a minor 

 matter and its cessation could not be followed by immediate loss 

 of the character whose appearance it had first caused. This theory 

 is in harmony with the known facts, but it remains untried and 

 is therefore only another possible explanation of evolutionary 

 processes. 



Evolution and the Internal Environment. Regardless of the 

 association of germinal and somatic genes, we must recognize 

 that the appearance of a new character in an organism, whatever 

 its cause, makes a change in the internal environment. The new 

 character can develop only if it receives the proper support of the 

 remainder of the organism. Consequently if it becomes large or 

 important, its new relationship to the internal environment may 

 become so fundamental that it will be of greater importance in 

 the expression of the character than the stimulus which originally 

 favored its development. 



In this secondary adjustment it is evident that structures 

 sometimes take on additional functions which make their per- 

 sistence necessary to the organism long after the loss of the original 

 use. Man has no further use for pharyngeal pouches as a part 

 of his respiratory system but the first pair have developed into 

 his middle ear and the remainder are associated with the formation 

 of a series of endocrine glands. They persist to the extent necessary 

 for the production of these structures. 



Experimental Evolution. The foregoing explanations are obvi- 

 ously not an adequate solution of the process of evolution, nor 

 are they intended to he. They show, however, that it is possible 

 to go much further in a logical evaluation of the factors in evolution 

 than has been done in the formulation of the famous theories of 



