Instinct is automatic obedience to the demands of conditions
external to the nervous system. As these conditions vary with
each kind of animal, so must the demands vary, and from this
arises the great variety actually seen in the instincts of different
animals. As the demands of life become complex, so do the in-
stincts. The greater the stress of environment, the more perfect
the automatism, for impulses to safe action are necessarily ade-
quate to the duty they have to perform. If the instinct were
inadequate, the species would have become extinct. The fact
that its individuals persist shows that they are provided with
the instincts necessary to that end. Instinct differs from other
allied forms of response to external condition in being hereditary,
continuous from generation to generation. This sufficiently dis-
tinguishes it from reason, but the line between instinct and reason
and other forms of reflex action cannot be sharply drawn.
It is not necessary to consider here the question of the origin
of instincts. Some writers regard them as “inherited habits,”
while others, with apparent justice, doubt if mere habits or
voluntary actions repeated till they become a “‘second nature”
ever leave a trace upon heredity. Such investigators regard
instinct as the natural survival of those methods of automatic
response which were most useful to the life of the animal, the
individual having less effective methods of reflex action perish-
ing, leaving no posterity.
Classification of Instincts. The instincts of fishes may be
roughly classified as to their relation to the individual into
egoistic and altruistic instincts.
