224 Heredity and Environment 



Nutrition and Development, Another type of functional ac- 

 tivity which is a potent factor in development is found in the 

 trophic or nutritive relations which exist between different parts 

 of the organism. Organs long unused undergo regressive changes 

 and may become rudimentary, for example the muscles of a limb, 

 which has been paralyzed or placed in a cast, shrivel; on the 

 other hand use increases the size and strength of any organ. In- 

 activity or atrophy of one part usually leads to imperfect nourish- 

 ment and development 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 gen- 

 eral the normal development of any part is dependent upon its 

 proper nutrition and this is dependent upon the functional activity 

 of this and other related parts. 



Internal Secretions; Hormones. Still another phase of func- 

 tional activity is found in the effects of certain secretions and 

 chemical substances which are formed by different glands. In 

 many cases the secondary sexual characters which distinguish the 

 male or the female are due to chemical substances from the 

 testes or the ovary, which stimulate or inhibit the formation of 

 these characters. If the ovary is removed from a young hen she 

 develops the larger size, the more brilliant plumage and the pecu- 

 liar comb, wattles and spurs of the cock. These secondary sexual 

 characters of the male are potential in the female but are kept 

 from developing or are inhibited by the activity of the ovary. 

 On the other hand the castration of the young cock does not 

 prevent the development of most of the secondary sexual char- 

 acters of the male. In the case of mammals removal of the ovar- 

 ies of a young female or of the testes of a young male does not 

 lead to the development of the secondary sexual characters of 

 the other sex, but both sexes remain in a sexually undeveloped or 

 infantile condition, that is, the presence of ovaries or testes serves 

 as stimulus to call forth the development of the secondary sexual 

 characters in mammals, and not as inhibitors to prevent the de- 

 velopment of the secondary sexual characters of the opposite sex, 



