y to suppose that the birds have any plan or mental image of
what the nest is to be like; probably not. But their state, in the
nest-building season, is such that they are impelled to build, and the
tendency is not quieted till the completed nest is there.

The mating instinct, in unsophisticated members of the human species,
is another perfect example. So is the hunting instinct in a dog; when
this instinct is aroused, the animal makes a lot of movements of
various sorts, responses to various particular stimuli, but evidently
these movements are not sufficient to quiet the tendency, for they
continue till the prey is captured. The behavior of a gregarious
animal when separated from his fellows shows the same sort of thing.
Take a young chick out of the brood and fence it away from the rest.
It "peeps" and runs about, attacking the fence at different points;
but such reactions evidently do not bring satisfaction, for it varies
them until, if a way out of the inclosure has been left, it reaches
the other chicks, when this series of acts terminates, and gives way
to something quite different, such as pecking for food.

The persisting tendency does not produce the series of movements all
by itself, but, as was explained in speaking of tendencies in general,
cooeperates with sensory stimuli in producing them. Clearly enough, the
nest-building bird, {111} picking up a twig, is reacting to that twig.
He does not peck at random, as if driven by a mere blind impulsion to
peck. He reacts to twigs, to the crotch in the tree, to the half-built
nest. Only, he would not react to these stimuli unless the nesting fit
were on him. The nest-building tendency favors response to certain
stimuli, and not to others; it facilitates certain reactions and
inhibits others. It facilitates reactions that are _preparatory_ to
the end-result, and inhibits others.

Fully and Partially Organized Instincts

Insects afford the best examples of very highly organized instincts.
Their behavior is extremely regular and predictable, their progress
towards the end-result of an instinct remarkably straightforward and
sure. They make few mistakes, and do not have to potter around. By
contrast, the instincts of mammals are rather loosely organized.
Mammals are more plastic, more adaptable, and at the same time less
sure; and this is notably true of man. It would be a mistake to
suppose that man has few instinctive tendencies; perhaps he has more
than any