ECONOMY OF THE BODY 143 



evolution of the hormones themselves we cannot go far, 

 but we must think of the body as a vast system of sym- 

 biotic organs, tissues, and cells. There is a continual 

 " pooling " of products, and there has doubtless been an 

 age-long elimination of those parts whose contributions 

 tended to endanger harmony and stability. It is probable 

 that some at least of the hormones are end-products 

 or by-products of a routine of metabolism which had 

 previously some less recondite significance, and it is 

 noteworthy that some of the hormones are produced 

 by structures {e.g. thyroid and pituitary) which have drifted 

 from their primary utiHsation. It may be as Professor 

 Starling suggests, that these end-product or by-product 

 hormones have been retained in the course of evolution 

 not so much because of any intrinsic philtre-like virtue in 

 themselves, but rather because certain other structures 

 happen to be peculiarly sensitive to their influence in the 

 way of either spur or bridle. 



