THE WORLD OF LIVING ORGANISMS 151 



In this way, after the one specific substance has been 

 formed at some part of the body, a certain cell at the other 

 end of the body, can, through the agency of the circulatory 

 system, be struck as precisely as the letters of the alphabet 

 on the typewriter. 



The part played in digestion by these substances cast into 

 the blood-stream — the so-called " hormones " — has only quite 

 recently been determined. But it has been known for a long 

 time that the breathing movements of vertebrates are kept in 

 full activity thanks to the stimulation of nerve-cells in the 

 central nervous system by the chemical products of tissue- 

 respiration. 



Further, there can be no doubt that the completely 

 different behaviour of animals in the states of hunger and 

 of satiety, is referable to change in the irritability of the 

 central organs belonging to the food-circle. For a newly- 

 fed shark a dead sardine simply is not there, because in this 

 condition the shark's " stimulus threshold " is too high. But 

 hunger lowers the stimulus threshold, and then the sardine 

 appears in the sensed-world of the shark. 



Steinach's remarkable experiments have shown what an 

 influence internal secretion exercises on the whole sexual life 

 of the higher animals. 



The purpose of this chemical organisation is to regulate 

 thoroughly the guidance of the central nervous apparatus 

 vis-a-vis of the influence of the indications. In comparison 

 with this, direct stimulation of the nervous system is relatively 

 negligible. For the most part, only a kind of " chemical tone " 

 is attained, which, in full conformity with plan, provides 

 that, according to the needs of the body, one function-circle 

 shall find acceptance rather than another, by the indications 

 thereof being more powerful or alone operative. Thus at 

 the breeding-season the prey-circle, and even the enemy- 

 circle, yield to the sexual circle, the indications of which 



