PLAN OF THE TREATISE. 5 



primary types, which we set out with supposing to 

 exist. 



(3.) Oiir introductory notices will contain, as formerly, 

 a rapid sketch of the chief peculiarities of these classes, 

 more especially in regard to their external anatomy j not 

 only as being that part of their structure most essential 

 for determining the species, genera, and families, but 

 also because it can be best understood and comprehended 

 by all. Our own classification, in fact, is as strictly 

 founded on anatomical organisation — even more so — 

 than those of any of our predecessors; with this differ- 

 ence only — that we have selected the more determinate 

 characters for the definition of our groups, rather than 

 resting them solely upon one or two. It. is a law of 

 nature, that the internal and external anatomy are mu- 

 tual indexes to each other; and, therefore, to give a 

 preference to internal characters (manifested only to the 

 view by skilful dissection) over such as are apparent 

 externally, has ever appeared to us not only objection- 

 able, but absolutely unnecessary. The study of zoology, 

 from its vast increasing extent and consequent difficulty, 

 stands in need of every help and of every facihty for its 

 acquirement. Comparative anatomy, indeed, is of the 

 highest importance in determining questions which 

 could otherwise not be solved ; but among vertebrated 

 animals, at least, this study seems to have been pushed 

 much too far ; and if ichthyological groups are to be 

 regulated by the bones of the head and the armature of 

 the mouth, we may, with equal propriety, draw up an 

 ornithological system from the structure of the wind- 

 pipe, the form of the sternum, or the number of the ver- 

 tebrae.* Were it possible to frame such systems — which 

 it manifestly is not — very many facts, of peculiar interest 

 to the mere comparative anatomist, would unquestionably 

 result. But the question arises, of what practical use 

 would they become ? The great mass of mankind look 



* Since this was written, a system, much on this plan, has actually been 

 but forth in one of our periodicals : the next month will probably bring out 

 another, founded on the structure of the gizzard, or the bones of the era- 

 nium, to add to the ninety and nine that have already died natural deaths. 



B 3 



