I have given, thus far, only a few very conspicuous examples of instinct, in order the more clearly to contrast this faculty with intelligence. I might have added many other conspicuous and well recognized examples, such as the migrations and nest-making of birds, the dam-building of beavers, etc. But we would have a very imperfect notion of the wideness of the operation of instinct if we confined ourselves to these conspicuous cases. Instinct in a less remarkable degree is universal among animals, including man himself. But what is universal in the popular mind creates no surprise, attracts no attention, and seems to need no explanation; yet it is these very universal and therefore unobserved phenomena which are the most instructive to science. Not only the action of bees and ants and wasps, not only the migrations and the nest-making of birds and the dam-building of beavers, must be accredited to instinct, but also all complex voluntary motions which are performed without experience. Such, for example, in many animals are the acts of running, swimming, flying, walking, and standing, etc. Yes, even the simple act of standing or walking is really a marvelous feat in balancing—requiring the nice adjustment and perfect coördination of perhaps a hundred different muscles. Even the simple voluntary act of sight (looking) requires the most exquisite adjustment of the optic axes, the lenses, and the iris. These complex actions are acquired by us by experience., though there is doubtless also, even in us, a large inherited element, an inherited capacity by which we acquire them with comparative facility. But the new-born ruminant quadruped or gallinaceous bird stands and walks and uses the eyes at once, without experience. The power to coördinate these muscles, and to accomplish these complex and difficult actions, is wholly inherited, not acquired.[1]
Thus defined, intelligence and instinct are not mutually exclusive, as some seem to suppose: the one is not simply a characteristic of man and the other of animals, but they coexist in varying relative proportions throughout the animal kingdom. As a broad general fact, in going down the animal scale we find that instinct varies inversely as intelligence.[2] The accompanying diagram expresses in a general way this relation. If the line a b represents the animal scale from
