for we know that
the distribution and existence of cattle and other animals in South America
absolutely depends 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 {196} former period, have been transmitted in nearly the same state,
although now become of very slight use; and any actually injurious
deviations in their structure will always 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 swimbladders 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 with the dog, though the aid must
be slight, for the hare, with hardly any tail, can double quickly enough.

In the second place, we may sometimes attribute importance to characters
which are really of very little importance, and which have originated from
quite secondary causes, independently of natural selection. We should
remember that climate, food, &c., probably have some little direct
influence on the organisation; that characters reappear from the law of
reversion; that correlation of growth will have had a most important
influence in modifying various structures; and finally, that sexual
selection will often have largely modified the external characters of
animals having a will, to give one male an advantage in fighting with
another or in charming the females. Moreover when a modification of
structure has primarily arisen from the above or other unknown causes, it
may at first have been of no advantage to the species, but may subsequently
have been taken advantage of by the