INFLUENCE OF ENVIRONMENT 329 



a potent factor in development is found in the 

 trophic or nutritive relations which exist be- 

 tween different parts of the organism. Or- 

 gans long unused undergo regressive changes 

 and may become rudimentary, for example 

 the muscles of a limb which has been par- 

 alyzed or placed in a cast shrivel ; on the other 

 hand use increases the size and strength of any 

 organ. Inactivity or atrophy of one part usu- 

 ally leads to imperfect nourishment and de- 

 velopment of related parts; for example, the 

 optic nerve atrophies when the eye is lost, and 

 muscles atrophy when the nerves leading to 

 them are destroyed or paralyzed. In general 

 the normal development of any part is de- 

 pendent upon its proper nutrition and this is 

 dependent upon the functional activity of this 

 and of other related parts. 



Still another phase of functional activity is 

 found in the effects of certain secretions and 

 chemical substances which are formed by dif- 

 ferent glands. In many cases the secondary 

 sexual characters which distinguish the male 

 and the female are due to chemical substances 

 from the ovary or the testes, which stimulate or 



