326 Problems of Organic Adaptation 



dent upon a proper sequence and balance of internal secre- 

 tions. Such a secretion known as "andrase," formed by the 

 interstitial cells of the testes, leads to the development of the 

 secondary sexual characters of the male in mammals, while 

 a corresponding hormone, known as "gynase," is formed by 

 the ovary and causes the development of the secondary 

 sexual characters of the female. The great changes which 

 accompany pregnancy and lactation in mammals are caused 

 by hormones from the ovary and the foetus. Other internal 

 regulations affecting many parts of the body and the general 

 course of development are caused by hormones from the 

 thyroid gland, the pituitary body and many other organs of 

 internal secretion. 



But although the hormone is the chemical stimulus which 

 leads to the adaptive reactions in each of these cases, it does 

 not in the least explain the fact that these reactions are 

 adaptive. Why should the reactions of so many different 

 organs to adrenin be of such a nature that they cooperate 

 to fit the animal for fight or flight? Why should the reac- 

 tions of so many different parts of the body to andrase or 

 gynase be of such a character that they lead to the develop- 

 ment of all the complicated organization of the male or 

 female, and why should the organization of the two sexes 

 be so adapted to each other? It is evident that the stimulus 

 which starts these adaptive reactions does not explain the 

 fact that they are adaptive. That can be found only in the 

 teleological nature of the mechanism which is set in motion 

 by these hormones. 



Finally, some of the most remarkable of all individual 

 adaptations are found in the regulations and regenerations 

 which follow injury. Many eggs> embryos, and adults have 

 the power of restoring lost parts and in general of resuming 

 their typical form after injury. Certain flat worms and 



