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 instances to illustrate these latter
remarks. 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 hide
this tree-frequenting bird from its enemies; and consequently that it
was a character of importance and might have been acquired through
natural selection; as it is, I have no doubt that the colour is due to
some quite distinct cause, probably to sexual selection. A trailing
bamboo 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, the hooks on the bamboo may have arisen from unknown laws
of growth, and have been subsequently taken advantage of by the plant
undergoing further modification and becoming a climber. The naked skin
on the head of a vulture is generally looked at 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. The sutures in the
skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has
been taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and
unimportant variations; and we are immediately
made conscious of this by reflecting on the differences in the breeds
of our domesticated animals in different countries, more especially in
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