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 
disturbance 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 
