64 DEVELOPMENT AND HEREDITY. 



tion of that organ. All researches into the history 

 of organic evolution, point to the conclusion that all 

 development has occurred gradually by slightly addi- 

 tional growth of various parts, in successive genera- 

 tions, — the different parts assuming new functions 

 as their changes of mass and form made the per- 

 formance of the new functions possible. 



When development occurs, therefore, it means 

 that growth takes place at a certain definite point. 

 Now what is the cause which determines that new 

 material shall be added at this particular point, rather 

 than anywhere else .? We have seen, in the previous 

 chapter, that a great variety of forces in the environ- 

 ment constantly act upon the organism and produce 

 a variety of effects ; but we cannot suppose that 

 these are the immediate causes of the complicated 

 processes of growth. Since the activity of the ner- 

 vous system regulates all the functions of the body 

 and controls its assimilation as a whole, we, must 

 suppose that it also controls the growth or develop- 

 ment of each part. The nervous system is, of. all 

 parts of an organism, the most sensitive to change, 

 and the forces of the environment must act chiefly 

 upon the nervous system, and through it upon the 

 whole organism. In view of all we know of the 

 nervous system and its functions, and from analogy 

 of its known control over other organic processes, we 



