natural selection, except by the 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 amongst extinct 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 antenn ; 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 antenn on the abdomen first
of one aphis and then of another; and each aphis, as soon as it felt
the antenn , immediately lifted up its abdomen and excreted a limpid
drop of sweet juice, which was eagerly devoured by the ant. Even the
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