DEVELOPMENT BY NATURAL SELECTION. 119 



and stability, such as the normal characteristics of an 

 organism have. These characteristics, whether of struc- 

 ture, or activity, or immediate environment, hang 

 together as a whole. We cannot alter them piece by 

 piece, simply because they hang together as a whole. 

 But an organism is bound up with its environment ; 

 and though it clearly tends to keep its environment 

 normal, and particularly its immediate environment — 

 for instance the blood, or the air in the lungs, or the 

 immediately available food supply in the alimentary 

 canal — this is never a perfect process. With any gross 

 or continuously acting change in external environment 

 the whole organism must vary to some extent. 



When we examine physiologically the variations which 

 occur in response to changes in environment we find 

 that they are " adaptive " changes. That is to say, 

 they are changes of a compensatory character, or of 

 such a nature that the general disturbances associated 

 with the altered environment are reduced to a minimum. 

 The existence of a tendency to compensatory adaptation 

 to all the accidents and ills of life is the great central 

 fact on which medicine and surgery are based. Their 

 function is to assist in this adaptation. Without its 

 existence they would be powerless. If wounds did not 

 tend to heal, and the body generally to adapt itself to 

 any remaining abnormality ; if damaged tissues did 

 not tend to repel the attacks of disease, and establish a 

 new normal, no medical or surgical skill would be of the 

 slightest use. And, similarly, if the body were not con- 

 stantly adapting itself to the indefinitely varying changes 

 in environment which are constantly occurring, life 

 would be non-existent. Life is in reality just continuous 



