or from being correlated with constitutional differences, 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, for
so trifling an object as driving 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
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 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;
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