earth. If not already become, the prototype was, therefore, certainly in the course of becoming, a true biped, using his legs alone for locomotion and developing perfectly plantigrade feet. Freed from further service for locomotive purposes, his arms had, no doubt, become relatively short and his hands exclusively prehensile. In all these respects the reconstructed skeleton of Pithecanthropus shows a striking divergence from the characteristic anthropoid type. The apes as a group are stoop-shouldered tree-dwellers, with hands and feet primarily adapted to swinging and climbing. Even the few varieties that dwell habitually upon the ground, of which the baboon is the best example, have not lost their inherited ability to climb, while in walking these creatures have reverted to the quadruped habit. Some of the arboreal apes also come occasionally to the ground; but when they walk, these animals adopt a crouching attitude and use their arms as aids to locomotion. The gibbon is, in fact, the sole ape that walks erect, and this is only possible in his case by reason of the extraordinary length of his arms. When unconcerned this creature walks with his long arms held aloft like balancing poles and hands outstretched to grasp any overhanging support, but when frightened the gibbon likewise drops his arms to the ground and swings along between them as if on crutches. Taking structural characteristics into account, it is, therefore, possible to draw a sharp distinction between the short-legged, long-armed, stoop-shouldered, arboreal ape, and the long-legged, short-armed, erect, surface-dwelling ancestor of man.
Specific variation in the biological sense is the accomplishment of that which variability permits, environment requires, and selection directs. No stock or lineage breeds perfectly true; the line of every descent is marked by certain modifications. The general tendency of such modification is toward the preservation of the more useful and the extinction of the less useful or useless characters. Survival is thus the result of the selection of such variations as adapt the organism to its environment, the more plastic the organism the greater the possibility of variation; the more favorable the environment the higher the type of animal evolved. Attributing variability, then, to the anthropoid stock, from the fact of man's survival, we may presume that the modifications characteristic of the human species were favorable in this sense, that they adapted the pliocene precursor to his mundane environment. Judging from the further fact that the tree-dwelling apes have long since reached the stationary stage, while land-dwelling man has steadily continued to advance, we are also justified in considering the terrestrial habitat more advantageous than the arboreal abode. The descent of man appears, accordingly, to be marked by progress along the line of heredity and improvement in the way of environment, selection in this instance giving rise to a distinctly superior species.