74? CLASSIFICATION OF FISHES. 



assigned to them. We have already shown, in former' 

 volumes, that the primary groups of birds represent 

 those of quadrupeds ; and it therefore follows, that if we 

 can find certain groups of fish which represent both 

 these, we arrive, by induction, to the sure conviction 

 that such groups of fish constitute the primary divisions 

 of the class. The Plectognathes, for instance, are placed 

 by M. Cuvier as a part of the osseous fishes, when, 

 even by his own admission, their skeleton is semi-car- 

 tilaginous. The group, however, is evidently natural ; 

 and we accordingly preserve it, giving it only a higher 

 rank. But this change^ however, is not the result of 

 arbitrary opinion : neither is it because the great fathers 

 of ichthyology did the same ; for they also were guided 

 in their decision, not by principle, but opinion. It is 

 because these fishes, besides the peculiarity of their 

 skeleton, unquestionably represent one of the grand 

 divisions of the Vertebrata, as well as one of the 

 primary orders of quadrupeds, of birds, and of reptiles : 

 and as there is no other division of fishes which 

 does the same, the Plectognathes are thus proved to be 

 one of the chief divisions of the class. The same 

 remark is applicable to the apodal fishes of authors, 

 where we find all the species destitute of ventral fins ; 

 but the skeleton is variable. Why, then, is this an order ? 

 The question is thus answered : Cuvier has shown they 

 are closely connected, and, in fact, pass into the osseous 

 fishes ; and he also coincides in the opinion of all our 

 best zoologists, that they likewise make an equally close 

 approximation to the Vermes, or worms. Now these 

 apparently opposite relations could not well be true, if 

 some of the eels had not the bony skeleton of the more 

 perfect tribes, while in others it was rudimentary, in 

 order to mark their proximity to the Vermes. The 

 skeleton is consequently variable ; but in all other respects 

 the characters of the apodal fishes are constant. 



(72.) Preserving the distinction between artificial 

 and natural systems elsewhere explained*, we shall at 



* Geography and Classification of Animals, p. 125. 



