CHAPTER II 
THE INFLUENCE OF EXTERNAL CONDITIONS IN CAUSING 
CHANGES IN THE STRUCTURE OF ANIMALS 
ANIMALS and plants are so constituted that one of their chief 
characteristics is that they respond to their natural environment 
in such a way as to insure their continued existence. These 
responses are in the main physiological, and therefore in large 
part transitory; but in some cases the response is structural, 
involving a temporary or even a permanent change in form or 
structure that persists at least so long as the external condi- 
tions that called it forth remain. The question arises whether 
these changes, directly induced by the environment, may not 
give origin to the more fixed characters that have become the 
permanent inheritance of each species. May not these have 
been in the first instance adaptive responses to the environ- 
ment? This leads to the further question ,of the origin of all 
the characters of the species, whether adaptive or non-adaptive. 
In this and in the following chapters the different sides of this 
question will be considered. 
ADAPTIVE RESPONSES 
External conditions sometimes cause adaptive structural 
changes in organisms. We are familiar with some effects of 
this sort in our own bodies. Pressure on the skin, if long con- 
tinued, causes it to become thicker and more capable of resist- 
ing the injurious effects of pressure. Sunlight tans the skin 
and protects it from “‘burning.” It is said that cold causes the 
fur of some mammals to become thicker, and this change better 
protects them against the cold. Conversely, it is said that horses 
and dogs lose their hair to some extent in very warm climates. 
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