40 Instincts, Habits, and Adaptations 
environment carried to the brain and then unconsciously re- 
flected back as motion. The impulse of fear is of the same 
nature. Reflex action is in general unconscious, but with ani- 
mals, as with man, it shades by degrees into conscious action, 
and into volition or action ‘‘done on purpose.” 
Instinct._—_Different animals show differences in method or 
degree of response to external influences. Fishes will pursue 
their prey, flee from a threatening motion, or disgorge sand or 
gravel swallowed with their food. Such peculiarities of dif- 
ferent forms of life constitute the basis of instinct. 
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. 
Egoistic instincts are those which concern chiefly the indi- 
vidual animal itself. To this class belong the instincts of feed- 
