In organic differentiation, then, man seems to have reached the highest possible point. Also, in avoidance of the constant forces of gravitation and friction, he has almost achieved perfection. And he starts life with the least expenditure of force in embryological development. In all these respects he seems to have attained the utmost height of organic development.
In respect to his adaptation to the other forces of nature—his powers of sensory perception—he is also in advance of all other animals. Not only is the division of labor of the animal organs within him—the nervous, muscular, and osseous systems—complete, but his exterior sensibility to the impress of force is the most delicate of that of all animals. This is perhaps not the case in the organs of special sense, though the position of the human nose, with its nostrils directly overhanging the mouth, seems a superior adaptation to its duty in the perception of odors. But in regard to the sense of touch, not only has he a superior provision in the tactile organs of the fingers, but the naked and soft condition of the skin renders it susceptible to contact in a degree not possessed by any other animals.
Other animals, in fact, are either covered with a dense coat of hair or feathers for protection from cold, or with a thick leathery or bony skin as armor against danger. In the development of man alone has Nature pursued her most elevated path, increasing his susceptibility to exterior influence, his power of gaining sensible experience of nature, to the utmost possible degree. This is probably the true explanation of the naked condition of the human skin. His mode of life has rendered the fullest perception of nature desirable, and adaptation has consequently taken this direction, removing from his exterior surface everything opposing the utmost sensibility, and, for the same reason, hindering any undue thickening or induration of the outer skin.
Such is man—the extreme upward limit of physical progress in organic nature—the one last step forward which living beings have taken after their long permanence in the quadrupedal stage. And beyond his form no physical progress seems possible, for he fulfills what we conceive to be Nature's design, viz., to husband force by the fullest avoidance of gravity and friction, to decrease weight to the lowest available point consistent with the size and strength necessary to best adapt him to surrounding conditions, and to produce the utmost susceptibility to impression of natural force—to attain a form, in fact, having the greatest excess of available energy, and best adapted to gain experiences of the conditions of nature.
But these very advantages in the human form produce certain un-
- In the prone vertebrate, the serpent, the escape from gravitation is accompanied by a marked increase of friction. In man both gravity and friction are simultaneously decreased to the greatest possible extent. Thus the serpent and man occupy the two poles in the development of motive powers, while all other vertebrates occupy intermediate positions.