254 THE AMERICAN BEAVER. 



life would be impossible; and yet, without the pres- 

 ence of a mental principle to take cognizance of the 

 impressions thus conveyed, their object would neces- 

 sarily be defeated. 



These intimacies of structure are particularly re- 

 markable among the mammals. The office and func- 

 tions of the several bones and muscles of the animal 

 frame are much the same in the different species. So 

 the nervous system, which is centralized in the brain, 

 is distributed throughout the body in such a manner 

 that the relative position as well as functions of its 

 several parts are similar, if not precisely the same, in 

 all. The several ganglia are found in the same con- 

 nection with the nerves of sensation and of motion, 

 and performing the same offices in a similar manner. 

 Such minute differences as exist find their explana- 

 tion in the special adaptation of each animal to his 

 sphere of life. In like manner, the circulating sys- 

 tem is constructed upon the same general plan, em- 

 ploying the same organs, with slight variations of 

 form. The same is equally true of the organs of res- 

 piration and of the digestive apparatus. One nomen- 

 clature suffices for the minutest subdivisions of the 

 mammalian form. The anatomist traces, with facil- 

 ity, this conformity of structure through all the 

 diversities which specific difference creates. Such 

 modifications of particular organs as occur are seen 

 to be necessary to meet special exigencies, such for ex- 

 ample as relate to subsistence and to motion. Thus, 

 the organs of respiration admit of considerable diver- 

 sity in size and form, according to the amount they 

 are required to furnish. Birds need a large quantity of 

 respiration to give to their muscles the strength, and 



