20 ESSAY ON CLASSIFICATION. Part I. 



field open for investigations in this most attractive branch of Zoiilogy. So much, 

 however, is ah'eady 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 homologies cannot be extended from one 

 type to the other, but are strictly lunited within each of them. The more remote 

 resemblance which may be traced between representatives of different types, is 

 founded upon analogy,^ and not upon affinity. While, for instance, the head of 

 fishes exhibits the most striking homology with that of reptiles, birds, and mammaha, 

 as a whole, as well as in all its parts, that of Articulata is only analogous to it and 

 to its part. What is commonly called head in Insects is not a head like that of 

 Vertebrata ; it has not a distinct cavity for the brain, separated from that which 

 communicates below the neck with the chest and abdomen ; its solid envelope does 

 not consist of parts of an internal skeleton, surrounded by flesh, but is formed of 

 external rings, like those of the body, soldered together ; it contains but one cavity, 

 which includes the cephalic ganglion, as well as the organs of the mouth, and all the 

 muscles of the head. The sanie may be said of the chest, the legs and wings, the 

 abdomen, and all the parts they contain. The cephalic ganglion is not homologous 

 to the brain, nor are the organs of senses homologous to those of Vertelirata, even 

 though they perform the same functions. The alimentary canal is formed in a very 

 different way in the embryos of the two tjqjes, as are also their respiratory organs, 

 and it is as unnatural to identify them, as it would be still to consider gills and 

 lungs as homologous among Vertebrata now embryology has taught us that in difter- 

 ent stages of growth these two kinds of respiratory organs exist in all Vertebrata in 

 very different oi'ganic connections one from the other. 



What is true of the branch of Articulata when compared to that of Vertebrata, 

 is equally true of the Mollusks and Eadiata when compared with one another or 

 with the two other types, as might easily be shown by a fuller illustration of the 

 correspondence of their structure, within these limits. This inequality in the fun- 

 damental character of the structure of the four branches of the animal kingdom 

 points to the necessity of a radical reform in the nomenclature of comparative 

 anatomy.^ Some naturalists, however, have already extended such comparisons 

 respecting the structure of animals beyond the limits pointed out by nature, when 

 they have attempted to show that all structures may be reduced to one norm, and 



^ See SwAixsox, (AV.,) On the Geoirrapln' and mologie? of Railiated Animals, with Reference to 



Classification of Animals, London, 18^5, l:imo., p. the Systematic Position of the Hydroid Polypi, 



129, whei-e this point is ahlj' discussed. Proc. of the Amer. Assoc, for the Adv. of Science 



^ See Agassiz, (L.,) On tlie Structure and Ho- for 1849, Boston, 1850, 1 vol. 8vo. p. 389. 



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