THE CHANGE OF HUMAN CHARACTER BY ENVIRONMENT 417 



of civilization can be easily explained even by taking only climate intC' 

 consideration. It is true that the people living in the regions of the- 

 climate lacking in the stimulating factors, especially in tropical regions, 

 seem to be born tired, whether or not it may be due to a disorder in 

 endocrine glands, and seem to dislike to use their brains. On the 

 other hand, disuse of the brain will result in its atrophy. Thus,, 

 disuse of the brain for successive generations may lead to the production 

 of mentally inactive people, whilst people live in climate full of sti- 

 mulating factors may become more and more mentally active by the 

 constant use of the brain. 



The change in the brain pattern thus produced may be transmitted 

 to germ cells by virus-like agents. In addition, hormones may be- 

 concerned with the transmission as in the pattern of other organs. 

 The majority of authors are inclined to believe that hormones exert 

 their most direct and pronounced effects upon behaviour through the 

 induction of changes in the central nervous system. As pointed out 

 already, the acquisition of new modes of behaviour as a result of 

 experience cannot occur in the absence of hormonal influences, although 

 all evidences point to the conclusion that capacity to learn is not 

 directly and immediately dependent upon the secretion of a single 

 endocrine product. 



In addition, certain hormones appear to exhibit pronounced effects 

 upon emotion in human (87). The conspicuous difference in human 

 emotions due to the difference in sex can, of course, be attributed to 

 the sexual hormones, indicating how great are the hormonal influences 

 upon the emotion. On the other hand, certain endocrine functions- 

 appear to be strongly influenced by psychic factors. Observing that 

 repeated gentling of rats changes their reactions towards man frorrr 

 flight or pugnacity to relaxation and docility, Hammctt (88) reports 

 that 79 per cent of "non-gentled" rats die within forty-eight hours 

 after parathyroidectomy, while among "gentled" individuals mortality 

 within the same period amounts to only 13 per cent. There is a 

 widespread impression that thyroid secretion may in certain instances 

 increase general irritability and contribute to the intensity of emotional 

 reactions. Administration of thyroid to Plymouth Rock hens is said 

 to result in a change from a gentle and phlegmatic to a highly ner\i0us 

 temperam.ent (89). In this connection, Rickey (90) claimed that the 

 offspring of rats which have been fed thyroid gland substance are 

 more nervous and erratic than the progeny of normal parents. On 

 the other hand, as will be mentioned later, adrenal extract exhibits 

 the function to release the nervous tem.perament in contrast to thyroid. 

 Thus it may be concluded that a change in the function of hormonal 

 glands due to the effect of environment, including climate, can induce 



