162 MAN --AN ADAPTIVE MECHANISM 



the uterus grow rapidly ; the mammary glands develop. 

 At this period of increased expenditure of energy for 

 physiological adjustment, there is a correspondingly 

 rapid growth in the thyroid, the adrenals and the 

 hypophysis. Without the normal development of the 

 adrenals, the thyroid and the hypophysis, neither the 

 male nor the female can develop secondary sexual char- 

 acteristics, show sexual desire, seasonal cycles of activ- 

 ity or power to procreate. On the other hand, all these 

 phenomena secondary sexual characters, sexual de- 

 sire, fertility can be created in the organism by feed- 

 ing the thyroid products of an alien species to the 

 individual deprived of a thyroid. 



At the close of the child-bearing period there is a 

 permanent diminution in the rate of energy conversion, 

 consistent with the fact that energy is no longer needed 

 for procreation and that less is required for self- 

 preservation. Unless other factors intervene, this re- 

 duction in the rate of energy transformation, together 

 with a corresponding reduction in the size of the thyroid 

 gland, is progressive until senescent death. 



The Purpose and the Mechanism of Heat Production 



As the organism transforms potential into kinetic 

 energy for the production of these various types of 

 visible and invisible motion, so it transforms energy for 

 the adaptive reaction of heat. 1 A by-production of 

 heat always accompanies every production of muscular 

 activity in the human body, but that this is not a use- 

 ful adaptation may be inferred from the fact that a 



1 The terms "heat" and "muscular action" are not used here in 

 a purely physical sense, but to designate physiological phenomena. 



