species, how very generally gradations, leading to
the most complex instincts, can be discovered. The canon of "Natura non
facit saltum" applies with almost equal force to instincts as to bodily
organs. 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 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 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 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 o