THE CYCLE OF LIFE 399 



it is well known that the internal secretions of the thyroid 

 gland which lies on each side of the larynx (or ' Adam's 

 apple ' ), and of the pituitary body (a remarkable organ 

 which is appended to the floor of the brain) have a specific 

 regulatory effect on the growth of the brain, the subcu- 

 taneous tissue, and the bones. It is said that a youth who 

 had been a successful candidate for a mihtary post, but was 

 debarred because of inadequate height, was able by a 

 judicious use of pituitary extract (obtained from ox and 

 sheep) to add in a few months the peremptorily required 

 cubit to his stature. It has been shown that the internal 

 secretions of the reproductive organs in vertebrate animals 

 have a specific effect on the growth of various parts of the 

 body, both of important organs, hke the milk-glands in 

 mammals, and trivial decorative structures, hke the comb 

 in poultry. It seems certain that some, if not all, human 

 giants are the result of the exaggerated secretion of the 

 pituitary body, and it is possible that some kinds of dwarfs 

 are due to a deficiency of the same stimulus. 



Even when we cannot at present suggest a physiological 

 interpretation, such as the influence of a specific secretion, 

 the fcLCt of the regulation of growth must be recognized. 

 Different parts grow at very different rates, yet the normal 

 result is proportionate growth. In cases of under-feeding, 

 there is great diversity in the effect on difierent organs ; 

 they do not suffer ahke. This points to a remarkable 

 internal regulation of growth. More familiar, and perhaps 

 simpler illustrations of the internal correlation may be 

 found in cases where an organ, such as the heart, responds 

 by increased growth to increased demands upon it. 



Galls are often formed by plants in response to some 

 external stimulus, such as the sahvary secretion of the larval 



