fore-limbs shall become useful only as grasping, the hind-limbs only as supporting organs. Necessarily, then, the pelvic joint must gradually change its direction from a right, angle to a straight line. The complete result, in fact, is only attained when the hind-limbs and the body form a vertical line, the function of support being performed entirely by these limbs, while the fore-limbs are freed for other functions, and so changed as to be specially adapted for grasping.
This desideratum is attained in man, and in man only of all the animal kingdom.
In the human form, then, we find all the advantages possessed by the mammalia as a class, together with certain important features of development not possessed by any other mammalian animal. Perhaps the most important of these is the fact that in man gravitation is overcome with a less expenditure of muscular force than in any other land animal. The whole weight of the body stands vertically above the organs of support. The muscles which in other animals act as ropes and levers of support are only called upon in man to preserve his vertical position. Evidently much less force is needed to preserve vertical equilibrium than to support horizontal weights.
The head, also, which needs muscular support in quadrupeds, in man presses directly downward upon the common center of gravity. And significantly the complete development of the brain tends to perfect this vertical position, as it yields a rounded and vertically poised Lead. The head in man has but one set of duties to perform, sensory and masticating labors. The hands bring food to it, instead of its having to seek food; therefore it has no need of the horizontal position and movements found in quadrupeds. Finally, that there shall be no weight needing muscular support, the fore-limbs hang vertically downward, being sustained by bones and tendons instead of muscles.
Support on the hind-limbs releases the fore-limbs to act as the defensive and offensive organs. For their most complete adaptation to this function the position of the shoulder-joint (like that of the pelvic joint) is changed, and the arms become lateral instead of ventral limbs. Finally, the teeth are released from duty as weapons, and are confined to their proper duty as masticating organs.
Thus only in man does the organic division of labor become complete, every function having a separate organ adapted to it alone. And the stores of force are husbanded to a degree not found in any other land-animal, the weight of the body being supported by bones instead of muscles, by adjusting instead of lifting energies.
And in regard to reproduction man surpasses all other animals, except the highest apes, in division of labor and localization of function. The young of the human race thus commence life with the best possible preparation, their vital activity being husbanded so that they enter upon their individual life-work at perhaps a higher starting-point than any other animal.