ances 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 the less civilized countries
where there has been but little artificial selection. Careful observers
are convinced that a damp climate affects the growth of the hair, and
that with the hair the horns are correlated. Mountain breeds always
differ from lowland breeds; and a mountainous country would probably
affect the hind limbs from exercising them more, and possibly even the
form of the pelvis; and then by the law of homologous variati