e slow and gradual accumulation of numerous, slight, yet
profitable, variations. Hence, as in the case of corporeal structures,
we ought to find in nature, not the actual transitional gradations by
which each complex instinct has been acquired--for these could be found
only in the lineal ancestors of each species--but we ought to find in
the collateral lines of descent some evidence of such gradations; or
we ought at least to be able to show that gradations of some kind are
possible; and this we certainly can do. I have been surprised to find,
making allowance for the instincts of animals having been but little
observed, except in Europe and North America, and for no instinct being
known among extinct species, how very generally gradations, leading to
the most complex instincts, can be discovered. Changes of instinct may
sometimes be facilitated by the same species having different instincts
at different periods of life, or at different seasons of the year, or
when placed under different circumstances, etc.; in which case either
the one or the other instinct might be preserved by natural selection.
And such instances of diversity of instinct in the same species can be
shown to occur in nature.

Again, as in the case of corporeal structure, and conformably to my
theory, the instinct of each species is good for itself, but has never,
as far as we can judge, been produced for the exclusive good of others.
One of the strongest instances of an animal apparently performing an
action for the sole good of another, with which I am acquainted, is that
of aphides voluntarily yielding, as was first observed by Huber, their
sweet excretion to ants: that they do so voluntarily, the following
facts show. I removed all the ants from a group of about a dozen aphides
on a dock-plant, and prevented their attendance during several hours.
After this interval, I felt sure that the aphides would want to excrete.
I watched them for some time through a lens, but not one excreted; I
then tickled and stroked them with a hair in the same manner, as well
as I could, as the ants do with their antennae; but not one excreted.
Afterwards, I allowed an ant to visit them, and it immediately seemed,
by its eager way of running about to be well aware what a rich flock it
had discovered; it then began to play with its antennae on the abdomen
first of one aphis and then of another; and each, as soon as it felt the
antennae, immediately lifted up its abdomen a