80 APPLICATION OF NATURAL SYSTEM. 



there is no impossibility in the possession of a scaly skin by an 

 air-breathing, warm-blooded animal; and we could not feel quite 

 certain that a Whale-like animai might not be hereafter dis- 

 covered possessing a scaly skin however improbable such a 

 thing may be. 



57. It should be the aim of the Zoologist, therefore, in the 

 choice of the characters which he uses, for the most easy dis- 

 tinction of the animals composing the several natural groups, 

 into which, (it cannot be too often repeated), they must be asso- 

 ciated by their general conformity of structure, to select those 

 which are the most natural, as indicating the nature of their in- 

 ternal structure, in preference to those which are artificial, giving 

 no information beyond that derived from themselves. Hence, in 

 classifying the Mammalia, the conformation of the teeth and ex- 

 tremities afford (in most orders at least) characters of the highest 

 value ; since these are intimately connected with the structure 

 of the digestive apparatus, the nature of the food on which the 

 animal lives, the mode in which it is obtained, and, consequently, 

 the entire habits of the species. And, in the subdivision of the 

 class of Birds, the conformation of the bill, wings, and claws, 

 afford characters of similar value. In the arrangement of Insects, 

 on the other hand, it is often necessary to adopt artificial charac- 

 ters for the separation of the several groups; because they are the 

 only ones which are recognised with sufficient facility, and be- 

 cause our comparative ignorance of their internal anatomy, as 

 well as of their economy, prevents us from understanding, as 

 clearly as in the two classes just mentioned, how far differences 

 apparently trifling in external conformation are essentially con- 

 nected with those peculiarities, which really characterise the re- 

 spective groups. Thus, the enormous order Coleoptwa (Beetles) 

 is subdivided into sections or sub-orders, according to the number 

 of joints in the tarsus, or foot ; a character which, at first sight, 

 appears very trivial, but which really does bring together the 

 families that have the greatest number of points of general agree- 

 ment, and separates them from others which differ more widely 

 from them. Hence it is probable, that an increased acquaintance 

 with the habits and structure of these animals will show us, why 



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