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. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as in the case of the dog, though the
aid in this latter respect must be slight, for the hare, with hardly any
tail, can double still more quickly.

In the second place, we may easily err in attributing importance to
characters, and in believing that they have been developed through natural
selection. We must by no means overlook the effects of the definite action
of changed conditions of life, of so-called spontaneous variations, which
seem to depend in a quite subordinate degree on the nature of the
conditions, of the tendency to reversion to long-lost characters, of the
complex laws of growth, such as of correlation, comprehension, of the
pressure of one part on another, etc., and finally of sexual selection, by
which characters of use to one sex are often gained and then transmitted