ds 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
the 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 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
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 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, etc., 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
descendants of the species under new conditions of life and with newly
acquired habits.
To give a few inst
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