XII. THE EVOLUTION OF MANKIND AND ITS FUTURE 423 



isolation of the hormone in a crystalline form from anterior pituitary 

 glands (93) (94). The administration of this hormone leads to weight 

 gain, muscle hypertrophy, increased skeletal dimensions, and skin thick- 

 ening in hypophysectomized rats. Climatic factors in general appear 

 to stimulate the function of pituitary gland, and hence people living 

 under a stimulating climate will increase in stature owing to the 

 increasd secretion of this growth hormone. 



On the contrary, people live in the climate lacking in the stimulating 

 factor may become smaller because of the want of the growth hormone. 

 In addition, it has been believed that adrenocorticotrophic horm.one 

 (ACTH) of hypophysis whose production is considered to be increased 

 by the stimulating agent is antagonistic to both gonadotrophic and 

 thyrotrophic hormones which are also produced by the anterior lobe of 

 pituitary gland (95) (96), so that the absence of stimulating climate 

 may increase the production of these latter principles. 



In man, hyperthyroidism is often followed by degeneration and atro- 

 phy of adrenal cortex. There is an evidence of a decrease in cortical 

 function after long-continued increase in thyroid activity. Moreover, 

 adrenal extract counteracts the effect of thyroxin administration 

 even in normal animals, reducing the increased loss of nitrogen and 

 lowering the increased pulse rate. In addition, it is observed that 

 corticosterone and desoxycorticosterone counteract the effects of mild 

 hyperthyroidism of the liver (97). Corticosterone is known to be able 

 to modify or correct the symptoms of hyperthyroidism. 



There are also many experimental evidences that administration of 

 cortisone or ACTH will suppress thyroid function (98). Similar findings 

 have been obtained under experimental conditions which are known 

 to be accompanied by increased secretion of cortical hormone including 

 injection of epinephrine or formalin and exposure to abnormal temper- 

 ature (99). 



On the other hand, it is well recognized that animal metamorphosis 

 fails to occur after thyroidectomy. Further, gonadotrophic principles 

 are known to lead to the development of precocious sexual maturity. 

 In this connection, it should also be mentioned that metamorphosis of 

 a South African frog is retarded by the administration of adrenal 

 extract or desoxycorticosterone acetate (100). Thus, if metamorphosis 

 is promoted by thyroid hormones small precocious animals may result, 

 whilst this may be prevented by adrenal cortex. Stockard, (101) in 

 his extensive studies of dogs and their hybrids, showed that dwarfs, 

 such as the dwarf bulldogs and other dwarf breeds, have a larger 

 thyroid per weight of dog than such normal breeds as the fox-hound 

 while the giant breeds have a relatively small thyroid. During preg- 

 nancy, a remarkable hypertrophy of pituitary gland is forthcoming, 



