352 THE AMERICAN NATURALIST [Vol. XLIX 



concerned are of the nature of slow chemical reactions. 

 The application of Van't Hoff's law in these instances 

 is sufficiently plain. 



Considering the processes in living matter, from this 

 point of view, we may gain some insight into the reason 

 why so many of the factors or conditions entering into 

 the reactions occurring in the body of a higher organism 

 should be kept as nearly constant as possible. 



2. The General Conditions of the Reactions in the Cells 

 In determining the velocity of a reaction, we may deter- 

 mine (1) what quantity of the reacting substances com- 

 bine or react in unit time, the usual method of the labora- 

 tory, as has been shown above, or (2) we may determine 

 what quantities of material must be added in unit time 

 to keep the reaction going at a constant rate. Kecalling 

 now the nearly constant factors in the higher mammalian 

 organism, the oxygen content, the temperature, and the 

 hydrogen ion concentration all varying within relatively 

 narrow limits, and the variations usually being in such a 

 direction as to get more material to an active or working 

 structure in unit time, we can see that there are certain 

 very effective devices for maintaining a reaction at a 

 constant speed, which are the counterparts of the appara- 

 tus employed in the chemical laboratory. But the mech- 

 anisms in the living organism are capable of regulating, 

 with a great degree of exactness, more conditions than 

 any artificial mechanisms so far devised in the laboratory 

 can control. 



In the evolution of the organism the development of 

 the various regulating mechanisms which we have de- 

 scribed has brought about a set of conditions which tend 

 to keep the environment surrounding the cells relatively 

 constant. The analogy beween the reactions in the cells 

 and the slow reactions of the physical chemist becomes 

 clear. The temperature of the body being constant, the 

 reactions in the cells, dependent as they are, upon a con- 

 stant supply of material, go on at a relatively constant 

 rate, or at such a rate as is determined by the needs of 



