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.