24 MIND IN EVOLUTION CHAP. 



which differentiates it from inanimate matter, the organism 

 preserves its own identity as clearly as the unchanging rock. 

 This feat of preserving itself in the midst of change could 

 not be accomplished as it is in the midst of ever varying 

 circumstances unless there were a certain equilibrium point, 

 as we may call it, which the organism is always striving to 

 maintain. It is true that the equilibrium is a moving 

 equilibrium. The process of rapid growth in youth, of 

 slow change during maturity, and of gradual decay ending 

 in death (or internal transformation ending in fission), is 

 of course the normal orbit of every organism. The equili- 

 brium point moves along this orbit, and the momentary 

 changes of which we speak are so many oscillations about 

 the equilibrium point as it moves. What immediately 

 concerns us is that every organism is so built, whether on 

 mechanical principles or not, that every deviation from the 

 equilibrium point sets up a tendency to return to it. This 

 is true both of the normal and of the abnormal circum- 

 stances that afreet the organism. A few illustrations may 

 make this general statement a little clearer. 



The normal breathing of the healthy man serves among 

 other things to maintain a certain balance between the 

 demand and supply of oxygen within the body. In 

 the alternate expansion and contraction of the lungs we 

 have a simple instance of oscillation about a mean point, 

 in which by whatever mechanism the deviation in one 

 direction itself tends to bring about the contrary motion. 

 Under certain circumstances, normal breathing does not 

 supply oxygen in sufficient quantities. If the atmosphere is 

 deficient in oxygen, or if by violent exercise the muscles 

 consume more than their due share, we begin to pant, to 

 take deeper breaths, and take them more quickly. The 

 same action has the effect of eliminating the extra quantity 

 of carbonic acid 1 evolved by the muscles during violent 

 exercise. The organism makes an effort to maintain as 

 nearly as possible its normal state. 



Still more instructive for our purposes is the mainten- 



1 It seems doubtful, however, whether the carbonic acid has a direct 

 effect. See Foster, Text Book of Physiology ', II. pp. 632 and 634, sixth 

 edition, 1895. 



