22 ESSAY ON CLASSIFICATION 



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 ani- 

 mals 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 limited within each of them. 

 The more remote resemblance which may be traced between repre- 

 sentatives 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 mammalia, 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 same 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 Vertebrata, even 

 though they perform the same functions. The alimentary canal is 

 formed in a very different way in the embryos of the two types, as are 

 also their respiratory organs, and it is as unnatural to identify them, 

 as it would be still to consider gills and limgs as homologous among 

 Vertebrata, now that Embryology has taught us that in different 

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

 Vertebrata in very different organic 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 Radiata when com- 

 pared with one another or with the two other types, as might easily 

 be shown by a fuller illustration of the correspondence of their struc- 

 ture within these limits. This inequality in the fundamental char- 

 acter of the structure of the four branches of the animal kingdom 

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



