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
more or less perfectly to the other sex, though of no use to the sex. But
structures thus indirectly gained, although at first of no advantage to a
species, may subsequently have been taken advantage of by its modified
descendants, under new conditions of life and newly acquired habits.
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