94 ORGANISM AND ENVIRONMENT 



We have seen that it is characteristic of an organ- 

 ism to react towards disturbing influences in such a 

 way as to maintain approximate constancy in its struc- 

 ture, internal environment, and even external environ- 

 ment. If the disturbance is merely slight, temporary, 

 and of normal occurrence, a simple and normal com- 

 pensating reaction occurs, and everything seems after- 

 wards to return again to its former state. But if the 

 disturbance is abnormal, or continued, a significant 

 fact emerges more and more clearly: for new and 

 apparently original compensatory reactions arise, or 

 an ordinary compensatory reaction is greatly strength- 

 ened, or supplemented. The new reaction is accom- 

 panied by corresponding structural change, which re- 

 mains to a greater or less extent after the cause of 

 distuibance has disappeared. 



We are now in contact with facts of a sort which 

 tend to lie in the background in connection with the 

 customary laboratory physiology of the present time, 

 but which spring into such prominence in common 

 everyday observation, and particularly in connection 

 with clinical medicine and surgery, as to make the 

 physiology of ordinary text-books appear somewhat 

 unreal. In the course of these lectures various facts 

 of the class here referred to have been described. 

 The Anglo-American expedition to Pike's Peak was 

 undertaken with the express object of ascertaining to 

 what extent, and in what manner, the body adapts 

 itself to a continued diminished concentration of oxy- 

 gen in the air breathed. The results showed that new 

 adaptations, apart from those demonstrable during 



