t 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 amongst 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 &c.; in which case either 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 with 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 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
{211} 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 aphis, 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. 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 a