native behavior, "an
instinct" is a unit of such behavior. Or, it is some unit of native
organization that equips the individual to behave in a certain way.
Different species of animals have different instincts, i.e., they are
differently organized by nature. The differences of organization lie
partly in the equipment of sense organs, partly in the equipment of
motor organs, and partly in the nerves and nerve centers that, being
themselves aroused by way of the sense organs, in turn arouse the
motor organs.

The dependence of instinct on sensory equipment becomes clear when we
think of animals possessing senses that human beings lack. The
instinct of dogs to follow the scent depends on their keen sense of
smell. Bees have something akin to a sense of taste in their feet, and
follow their own trails by tasting them. Fishes have special sense
organs along their sides that are stimulated by water currents, and it
is in response to this stimulus that the fish instinctively keeps his
head turned upstream.

The dependence of instinct on motor equipment is still more obvious.
The flying instinct of birds depends on the possession of wings, and
the swimming instinct of the seal depends on the fact that his limbs
have the peculiar form of flippers. The firefly instinctively makes
flashes of light, {107} and the electric eel instinctively discharges
his electric organ and gives his enemy a shock.

But the core of an instinct is to be sought in the nerve centers,
since it is there that the cooerdination of the muscles is
accomplished. A wing or flipper would be of no use unless its muscles
were excited to action by the nerve centers, and it would be of very
little use unless the nerve centers were so organized as to arouse the
muscles in a certain combination, and with a certain force and rhythm.
In terms of the nervous system, an instinct is the activity of a team
of neurones so organized, and so connected with muscles and sense
organs, as to arouse certain motor reactions in response to certain
sensory stimuli.

The Difference Between an Instinct and a Reflex

What we have said regarding instinct thus far could equally well be
said of reflex action. A reflex is a native reaction, and it is taken
care of by a team of neurones in the way just stated. We might speak
of a reflex as "instinctive", using this adjective as equivalent to
"native"; but we should shrink for some reason from speaking of the
pupillary reflex to li