CHARACTERISTICS: RELATION TO ENVIRONMENT 239 



physiology, which is enlarging rapidly. It all illustrates the 

 fact that control of development and of characteristics is 

 in large measure possible, and even at the present time is 

 in a considerable degree practicable. 



All together, therefore, it is clear that the processes of 

 development and the determination of the nature of char- 

 acteristics are not shut off from outside influence, that, on 

 the contrary, adjustment to the conditions is one of the 

 fundamental features of development. 



Thus far we considered mainly the role of environmental 

 conditions in determining the different characteristics of 

 different parts of the body. Do environmental conditions 

 likewise play a role in determining the different character- 

 istics of different individuals, so that individuals that have 

 developed under different conditions have different charac- 

 teristics? From what has just been set forth on the nature 

 of development, this might be expected. We have seen that 

 some of the characteristics developed depend on the hor- 

 mones that are produced within the body. It is known that 

 the production of some hormones is under the influence of 

 the nervous system, and through this, under the influence 

 of outer conditions. This is the case in man with the hor- 

 mone from the suprarenal capsules. This hormone affects 

 behavior, but little is known as to its effect on development. 

 Other hormones however are known to affect development 

 profoundly. Hormones from the hypophysis have a strik- 

 ing effect on growth. Differences in the quality and quantity 

 of these hormones in different individuals result in the pro- 

 duction of giants in some cases, of dwarfs in others, of 

 normal growth in still others. 



In another vertebrate a striking effect on development, 

 altering all the adult characteristics, is known to be pro- 

 duced through the action of a hormone. The axolotl (figure 

 48, A) is a large salamander that has large red external 



