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 wallowing in putridity; and so it may be, or it
may possibly be due to the direct action of putrid matter; but we should be
very cautious in drawing any such inference, when we see that the skin on
the head of the clean-feeding male turkey is likewise naked.
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