THE NEW PHYSIOLOGY. 75 



constant in composition. Were it not so the reactions 

 of the cells would become chaotic, and their structure 

 would be completely altered, if not destroyed. But the 

 constancy of the blood is maintained by the combined 

 reactions of the organs and tissues themselves. The 

 intimate structure of the living cells depends on the con- 

 stancy of the blood, and the constancy of the blood 

 depends on the intimate structure of the tissues. If we 

 regard this condition as simply a physical and chemical 

 state of dynamic balance, it is evident that the balance 

 must be inconceivably complicated, and at the same 

 time totally unstable. If at any one point in the 

 system the balance is disturbed, it will break down, and 

 everything will go from bad to worse. 



A living organism does not behave in this way : for 

 its balance is active, elastic, and therefore very stable. 

 When a disturbance affects its structure or internal 

 environment, it tends to " adapt " itself to the disturb- 

 ance. That is to say, its reactions become modified in 

 such a manner that the normal is in essential points 

 maintained. An injury heals up : destroyed tissue is 

 reproduced, or other parts take on its function : the 

 attacks of micro-organisms are not only repelled, but 

 immunity to future attacks is produced. In reproduction 

 the body periodically proceeds to renew almost the whole 

 of its structure. Death may be regarded as a periodical 

 scrapping of structural machinery, and reproduction as 

 its complete renewal. 



The Anglo-American expedition of which I was a 

 member studied, on the summit of Pike's Peak, Colorado, 

 adaptation to the want of oxygen which causes, in 

 unadapted persons, all the formidable symptoms known 



