In a former chapter I have given instances of very
trifling characters, such as the down on fruit and the colour of its flesh,
the colour of the skin and hair of quadrupeds, which, from being correlated
with constitutional differences, or from determining the attacks of
insects, might assuredly be acted on by natural selection. The tail of the
giraffe looks like an artificially constructed fly-flapper; and it seems at
first incredible that this could have been adapted for its present purpose
by successive slight modifications, each better and better fitted, for so
trifling an object as to drive away flies; yet we should pause before being
too positive even in this case, for we know that the distribution and
existence of cattle and other animals in South America absolutely depend on
their power of resisting the attacks of insects: so that individuals which
could by any means defend themselves from these small enemies, would be
able to range into new pastures and thus gain a great advantage. It is not
that the larger quadrupeds are actually destroyed (except in some rare
cases) by flies, but they are incessantly harassed and their strength
reduced, so that they are more subject to disease, or not so well enabled
in a coming dearth to search for food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted to existing species in nearly the
same state, although now of very slight use; but any actually injurious
deviations in their structure would of course have been checked by natural
selection.
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