292 RAMBLES OF A NATURALIST. 



reason. That magnificent a priori mode of conclu- 

 sion, which may be true as long as it is limited to 

 the Vertebrata, becomes in many cases strikingly 

 inexact when it is extended to the Invertebrata, and 

 more especially to the most degraded representatives 

 of this great sub-kingdom. To those who would 

 adopt literally, in the strict sense of the word, all 

 that Cuvier has stated in respect to respiration and 

 the respiratory organs, the B ranch ellion would in 

 itself form a distinct class.* To a follower of Jus- 

 sieu it would be merely the type of a sub-class, and 

 this is the point of view which we ourselves incline 

 to adopt. The nomenclature will thus express, if we 

 substitute value for number of characters, the resem- 

 blances which connect the Branchellion with the 

 other Hirudinea3 and the differences which distinguish 

 it from them. 



At the risk of appearing somewhat too technical, 

 I have thought it necessary to present my readers 

 with the preceding anatomical and physiological de- 

 tails. I have deemed it necessary to show at some 

 length how the minute examination of a single 

 animal, if the selection be a good one, leads us to the 

 most varied and delicate questions of zoology. I 



* I ought, however, to observe that Cuvier himself was always 

 most anxious to avoid this strict interpretation of his views. In this 

 great man pretensions to infallibility and a systematising spirit never 

 took the place of the most perfect good faith, and that sound common 

 sense which is one of the attributes of true genius. Thus, more 

 especially in the classification of the Annelids, he has not hesitated 

 to give way to facts in preference to the rules which he had himself 

 established, and this is certainly in a great measure owing to the 

 circumstance that he never confounded classification and method. 



