il 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.

If green woodpeckers alone had existed, and we did not know that there
were many black and pied kinds, I dare say that we should have thought
that the green colour was a beautiful adaptation to conceal this
tree-frequenting bird from its enemies; and consequently that it was
a character of importance, and had been acquired through natural
selection; as it is, the colour is probably in chief part due to sexual
selection. A trailing palm in the Malay Archipelago climbs the loftiest
trees by the aid of exquisitely constructed hooks clustered around the
ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees
which are not climbers, and which, as there is reason to believe from
the distribution of the thorn-bearing species in Africa and South
America, serve as a defence against browsing quadrupeds, so the spikes
on the palm may at first have been developed for this object, and
subsequently have been improved and taken advantage of by the plant, as
it underwent further modification and became a climber. The naked skin
on the head of a vulture is generally considered as a direct adaptation
for wa