THE DESCENT OF MAN. 367 



earth. If not already become, the prototype was, therefore, certainly 

 in the course of becoming, a true biped, using his legs alone for loco- 

 motion 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 loco- 

 motion. 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 over- 

 hanging 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. Xo 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 pos- 

 sibility 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 modi- 

 fications characteristic of the human species were favorable in this 

 sense, that they adapted the pliocene precursor to his mundane environ- 

 ment. 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. 



