448 V. THE NATURE OF EVOLUTION 



State when the gene falls in with some other genes having the original 

 structure. Therefore, in order to establish a peculiar, gradual change 

 the gene must be isolated from other genes having the original 

 structure. This may be the reason why isolation is needed for the 

 evolution. 



When a gene has achieved a gradual change to such an extent 

 that the genes having the original structure can no more exert their 

 influence upon it, the organism having the gene may be called new 

 species. Since changes of a gene, not great enough to reject the in- 

 terference of the unchanged, original gene, are always to be returned 

 to the previous structure by the latter, there should usually be a dis- 

 tinct line of demarkation between any two species, no gradual trans- 

 mission from a species to another being found among individuals exist- 

 ing without separation. 



It seems probable that the fate of civilization is subjected to 

 climate as Huntington believed. The want of stimulating effect in 

 climate may cause the loss of mental and physical activity in man, 

 leading to the disuse of brain and related organs, resulting in the 

 degeneration of the organs. 



Stimulating climates exert their influence on hypophysis, probably 

 in the main through autonomic nervous systems, and as a result the 

 secretion of corticotrophic and growth hormones may be enhanced at 

 the expense of thyrotrophic and gonadotrophic principles. Consequently, 

 under a stimulating climate man may tend to be increased in his 

 stature owing to the growth hormone, and at the same time his sus- 

 ceptibility to tuberculosis may be reduced, since status thymico-lym- 

 phaticus and cortical insufficiency, both of which raise the suscepti- 

 bility, are removed by the pituitary hyperfunction. 



It is known that civilized people have been showing the tendency 

 to be increased in their stature since about a hundred years, while on 

 the other hand, during the same period the resistance to tuberculosis 

 has been increasing steadily in them. These two amazing facts can 

 be readily explained by the assumption that the stimulating effect of 

 climate on hypophysis was heightened rapidly. 



This heightening of the effect, however, is probably not attributed 

 to the increase in the stimulating action of climate itself, but to the 

 increase in the susceptibility of man to the climate. The promotion of 

 civilization may enable man to live an easy life with a plenty of 

 nutrition. The cause of the increase in the susceptibility to climate 



