IN THE LIMBS OF MAMMALIA. 57 



aerial Vertebrates, their very bones filled with the medium in which they dwell, and always 

 in active motion ; and the cycle is again completed by the Mammals, which embody a more 

 equal and harmonious combination of all the systems of organs, living, like the Reptiles, 

 on the earth, but elevated above it into the free space of air. 



Professor Guyot considered the same law to prevail throughout all the lesser groups 

 of Mammalia, and also in the three other types ; and it is certainly worthy of remark, that 

 in these latter are recognized only three subdivisions, while in the Vertebrates four are gen- 

 erally acknowledged ; and so in every group containing man would be four lesser groups, 

 but in all others only three, the highest being wanting, and the series thus incomplete. 



In this connection circle seems as proper as series ; for the most natural exhibition of the 

 relative standing of the four types is by four equidistant points on the circumference of a 

 circle, the Radiates below, the Mollusks above and to the left, the Articulates the same 

 distance above and to the right, and the Vertebrates at the top ; the two intermediate 

 groups, the types of partial progress, being at the same distance from the lowest and 

 highest, and thus of equal rank, though opposite nature. Nor can we overlook the fact that 

 there are four regions of the body, pelvis, abdomen, thorax, and head ; and that, as I think 

 Oken has said, Fishes are pelvic animals, Reptiles abdominal, Birds thoracic, and Vertebrates 

 cephalic ; and that neither Radiates nor Mollusks nor Articulates possess a distinct anterior 

 segment containing any such overruling portions of the nervous system as does the head 

 of the Vertebrates. 



There are four senses also, one general, the others special. 



The sense of touch is universal, and only more or less developed in different regions of 

 the body : it is most exquisitely perceived through the agency of water or moisture, 

 especially in the tongue ; for taste is a peculiar exaltation of the general sensibility, and 

 forms, as it were, a transition therefrom to the smell, one of the three special senses with 

 which, as anatomy clearly shows, it cannot be allied. Of these latter, smell is the lowest, 

 and its exercise depends upon the presence of odoriferous particles of an earthy, solid 

 nature. Hearing is the second special sense, and perceives vibrations in the atmosphere, 

 the next higher medium ; while sight, the highest, depends for the exercise of its functions, 

 not upon this, but upon that invisible and imponderable yet material medium which is 

 called ether. 



Again, sight, is directly related to the central nervous system, and properly belongs to the 

 head, below which its organ does not extend. Hearing goes lower down, into the pharynx, 

 with which its organ is connected, anatomically by the Eustachian tube and physiologically 

 when we listen to the speech of another. Smell and its accessory taste preside over the 

 entrance to the alimentary canal, with which they descend through the head and thorax 

 into the abdomen. Finally, in the pelvis is the organ which, as will be hereafter shown, is 

 the posterior or reversed repetition of the tongue, and whose sensitiveness, like taste, is only 

 a peculiar exaltation of the universal sense, the nerves in both cases being the common 

 cranial or spinal nerves, and not special prolongations of the brain into the organ, as with 

 the eye, the ear, and the nose. 



Professor James D. Dana, in a paper " On the higher Subdivisions in the Classification of 

 Mammalia,*' (American Journal of Science and Arts, vol. xxxv. January, 1863,) proposes a 

 similar quaternary division of that class with special reference to the "cephalization" of 

 the body, which he shows to increase as we ascend in the scale. 



To those who make classification their study it belongs to decide how much influence 

 these principles exert among the lower groups ; but certainly among the higher ones the 



MEMOIRS HOST. SOC. NAT. HIST. Vol. I. 15 



