12 Evolution and Adaptation 
Figure 1 A, and at the same time the terminal flap bends over 
the open mouth of the throat, “stopping the entrance to the 
flies, which have now nothing more to do here.” 
ADJUSTMENTS OF THE INDIVIDUAL TO CHANGES IN THE 
ENVIRONMENT 
The most familiar cases of adjustments of the individual to 
the environment are those that we recognize in our own 
bodies. After violent exercise we breathe more rapidly, and 
take deeper inspirations. Since during exercise our blood 
loses more oxygen and takes in more carbon dioxide from the 
muscles, it is clear that one result of more rapid breathing is 
to get more oxygen into the blood and more carbon dioxide 
out of it. The process of sweating, that also follows exercise, 
may be also looked upon as an adaptive process, since by 
evaporation the skin is kept cooler, and, in consequence, the 
blood, which at this time flows in larger quantities to the skin, 
is cooled also. 
More permanent adaptive changes than these also take 
place as the result of prolonged use of certain parts. If the 
muscles work against powerful resistance, they become larger 
after several days or weeks, and are capable of doing more 
work than at first. Conversely, when any group of muscles is 
not used, it becomes smaller than the normal and capable of do- 
ing less work. It would be a nice point to decide whether this 
latter change is also an adaptation. If so it is one in a some- 
what different sense from that usually employed. The result 
is of no direct advantage to the animal, except possibly in sav- 
ing a certain amount of food, but since the same change will 
take place when an abundance of food is consumed, the result 
is, under these conditions, of no use. 
The thickening of the skin on those parts of the body where 
continued pressure is brought to bear on it is a change in a 
useful direction. The thickening on the soles of the feet and 
