26 ESSAY ON CLASSIFICATION. 



hind extremities. Quite as striking a coincidence is ob- 

 served between the solid skull-box, the immoveable bones 

 of the face, and the lower jaw of man and the other 

 mammalia, and the structure of the bony frame of the 

 head of birds, turtles, lizards, snakes, frogs and fishes. 

 But this correspondence is not limited to the skeleton ; 

 every other system of organs exhibits in these animals 

 the same relations, the same identity in plan and struc- 

 ture, whatever be the differences in the form of the parts, 

 in their number, and even in their functions. Such an 

 agreement in the structure of animals is called their 

 honiology, and is more or less close in proportion as the 

 animals in which it is traced are more or less nearly 

 related. 



The same agreement exists between the different sys- 

 tems and their parts in Articulata, in Mollusks, and in 

 Radiata, only that their structure is built upon respect- 

 ively different plans, though in these three types the 

 homologies have not yet been traced to the same extent 

 as among Vertebrata. There is, therefore, still a wide 

 field open for investigations in this most attractive branch 

 of Zoology. So much, however, is already plain, from 

 what has been done in this department of our science, 

 that the identity of structure, among animals, does not 

 extend to all the four branches of the animal kingdom ; 

 that, on the contrary, every great type is constructed 

 upon a distinct plan, so peculiar, indeed, that homo- 

 logies cannot be extended from one type to the other, 

 but are strictly limited within each of them. The more 

 remote resemblance which may be traced between repre- 

 sentatives of different types is founded upon analogy, 1 



1 See SWAINSON (W.), On the Geo- London, 1835, 12mo., p. 129, where 

 graphy and Classification of Animals; this point is ably discussed. 



