It was urged that the action of "like conditions,"
with "accidental resemblances" and "reversion to ancestral types," would
account for the facts. If, however, we consider the actual phenomena as
here set forth, and the very constant conditions under which they occur,
we shall see how utterly inadequate are these causes, either singly or
combined. These constant conditions are--
1. That the imitative species occur in the same area and occupy
the very same station as the imitated.
2. That the imitators are always the more defenceless.
3. That the imitators are always less numerous in individuals.
4. That the imitators differ from the bulk of their allies.
5. That the imitation, however minute, is _external_ and
_visible_ only, never extending to internal characters or to
such as do not affect the external appearance.
These five characteristic features of mimicry show us that it is really
an exceptional form of protective resemblance. Different species in the
same group of organisms may obtain protection in different ways: some by
a general resemblance to their environment; some by more exactly
imitating the objects that surround them--bark, or leaf, or flower;
while others again gain an equal protection by resembling some species
which, from whatever cause, is almost as free from attack as if it were
a leaf or a flower.
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