We should remember that climate, food, etc., probably have
some little direct influence on the organisation; that characters
reappear from the law of reversion; that correlation of growth will
have had a most important influence in modifying various structures;
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.
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