Nothing better illustrates this distinction than the difference between the fore foot of animals and the human hand. The first begins as a fin or paddle or is armed with a hoof, and is used solely for locomotion. Some carnivora with claws use the fore limb also for holding well as tearing, and others for digging. Arboreal life seems to have almost created the simian hand and to have wrought a revolution in the form and use of the forearm and its accessory organs, the fingers. Apes and other tree-climbing creatures must not only adjust their prehensile organ to a wide variety of distances and sizes of branches, but must use the hands more or less freely for picking, transporting, and eating fruit; and this has probably been a prime factor in lifting man to the erect position, without which human intelligence as we know it could have hardly been possible. "When we attempt to measure the gap between man and the lower animals in terms of the form of movement, the wonder is no less great than when we use the term of mentality."[3] The degree of approximation to human intelligence in anthropoid animals follows very closely the degree of approximation to human movements.
The gradual acquirement of the erect position by the human infant admirably repeats this long phylogenetic evolution.[4] At first the limbs are of almost no use in locomotion, but the fundamental trunk muscles with those that move the large joints are more or less spasmodically active. Then comes creeping, with use of the hip muscles, while all below the knee is useless, as also are the fingers. Slowly the leg and foot are degraded to locomotion, slowly the great toe becomes more limited in its action, the thumb increases in flexibility and strength of opposition, and the fingers grow more mobile and controllable. As the body slowly assumes the vertical attitude, the form of the chest changes till its greatest diameter is transverse instead of from front to back. The shoulder-blades are less parallel than in quadrupeds, and spread out till they approximate the same plane. This gives the arm freedom of movement laterally, so that it can be rotated one hundred and eighty degrees in man as contrasted to one hundred degrees in apes, thus giving man the command of almost any point within a sphere of which the two arms are radii. The power of grasping was partly developed from and partly added to the old locomotor function of the fore limbs; the jerky aimless automatisms, as well as the slow rhythmic flexion and extension of the fingers and hand, movements which are perhaps survivals of arboreal or of even earlier aquatic life, are coördinated; and the bilateral and simultaneous rhythmic movements of the heavier muscles are supplemented by the more finely adjusted and specialized activities which as the end of the growth period is approached are determined less by heredity and more by environment. In a sense, a child or a man is the sum total of his movements or tendencies to move; and nature and instinct chiefly determine the basal, and education the accessory parts of our activities.