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 and excreted a limpid drop of sweet juice, which was eagerly
devoured by the ant. Even the quite young aphides behaved in this manner,
showing that the action was instinctive, and not the result of experience.
It is certain, from the observations of Huber, that the aphides show no