MAN AND THE VERTEBRATE SERIES. 795 



In organic differentiation, then, man seems to have readied the 

 highest possible point. Also, in avoidance of the constant forces of 

 gravitation and friction, he bas almost achieved perfection.* And he 

 starts life with the least expenditure of force in embryological devel- 

 opment. 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 pow- 

 ers of sensory pei'ception 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 over- 

 hanging the mouth, seems a superior adaptation to its duty in the per- 

 cejation 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 i^ower 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 ren- 

 dered 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 be- 

 yond 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 sus- 

 ceptibility 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 de- 

 creased 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 intermedi- 

 ate positions. 



