quite young aphides behaved in this manner, showing that the action was
instinctive, and not the result of experience. But as the excretion is
extremely viscid, it is probably a convenience to the aphides to have
it removed; and therefore probably the aphides do not instinctively
excrete for the sole good of the ants. Although I do not believe that
any animal in the world performs an action for the exclusive good of
another of a distinct species, yet each species tries to take advantage
of the instincts of others, as each takes advantage of the weaker
bodily structure of others. So again, in some few cases, certain
instincts cannot be considered as absolutely perfect; but as details on
this and other such points are not indispensable, they may be here
passed over.
As some degree of variation in instincts under a state of nature, and
the inheritance of such variations, are indispensable for the action of
natural selection, as many instances as possible ought to have been
here given; but want of space prevents me. I can only assert, that
instincts certainly do vary for instance,
the migratory instinct, both in extent and direction, and in its total
loss. So it is with the nests of birds, which vary partly in dependence
on the situations chosen, and on the nature and temperature of the
country inhabited, but often from causes wholly unknown to us: Audubon
has given several remarkable cases of differences in nests of the same
species in the northern and southern United States. Fear of any
particular enemy is certainly an instinctive quality, as may be seen in
nestling birds, though it is strengthened by experience, and by the
sight of fear of the same enemy in other animals. But fear of man is
slowly acquired, as I have elsewhere shown, by various animals
inhabiting desert islands; and we may see an instance of this, even in
England, in the greater wildness of all our large birds than of our
small birds; for the large birds have been most persecuted by man. We
may safely attribute the greater wildness of our large birds to this
cause; for in uninhabited islands large birds are not more fearful than
small; and the magpie, so wary in England, is tame in Norway, as is the
hooded crow in Egypt.
That the general disposition of individuals of the same species, born
in a state of nature, is extremely diversified, can be shown by a
multitude of facts. Several cases also, could be given, of occasional
and strange habits in certain species, which might, if advantageous to
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