NOMENCLATURE. 141 



the danger of trusting to analogy, since very remarkable 

 differences may prevail, even where the points of resem- 

 blance are very numerous, and justifies us in concluding, that 

 in a natural genus there are artificial combinations. 



In the formation of classes and orders from the charac- 

 ters furnished by one system of organs, arbitrarily selected, 

 we are in danger of being misled, by considering all the 

 parts of that system of equal importance to its functions. 

 This, however, is very far from being the case. There is 

 frequently an intermixture of the organs of different sys- 

 tems, so that we are apt to be led away from the one we 

 resolved to employ, and to use the characters of the 

 organs of another system, which, even in their most perfect 

 form, we had deliberately rejected. Thus, in examining 

 the gills of fishes, we may confine our observation to the 

 number, extent of surface, and mode of attachment of the 

 red parts, which are subservient to the purposes of respi- 

 ration. But if we extend our observations to the central 

 surface of the arches of those with free gills, we enter the 

 sphere of the organs of deglutition. Errors of this kind 

 are frequently committed even by experienced observers. 



The Natural Method of classifying animals by the dif- 

 ferent systems of organs they possess, being inconvenient 

 in its application, and the Artificial Method, consisting in 

 the exclusive employment of any one system of organs, 

 producing incongruous combinations, a Mixed Method has 

 been adopted by many naturalists, which appears to answer 

 every useful purpose. The characters which are employ- 

 ed in the construction of this last method, are derived from 

 all the systems of organs, and the subordinate parts of 

 these, assigning as the test of their importance, the extent 

 of their occurrence and of their influence. 



The characters which are of universal occurrence, are 

 to be considered as the distinguishing marks of the subjects 



