Chap. II. CLASSES OF ANIMALS. 145 



SECTION II. 



CLASSES OF ANIMALS. 



Before Cuvlcr had shown that the whole animal kingdom is constructed upon 

 four different plans of structure, classes were the highest grou2)s acknowledged in 

 the systems of Zoology, and naturalists very early imderstood upon what this kind 

 of division should be founded, in order to be natural, even though in practice 

 they did not always perceive the true value of the characters upon which they 

 established their standard of relationship. Linnaeus, the first expounder of the 

 system of animals, already distinguishes, by anatomical characters, the classes he 

 has adopted, though very imperfectly; and ever since, systematic writers have aimed 

 at drawing a more and more complete picture of the classes of animals, based 

 upon a more or less extensive investigation of their structure. 



Structure, then, is the watchword for the recognition of classes, and an accurate 

 knowledge of their anatomy the surest way to discover their natural limits. And 

 yet, with this standard before them, naturalists have differed, and differ still greatly, 

 in the limits they assign to classes, and in the number of them they adopt. It 

 is really strange, that, applying apparently the same standard to the same object.s, 

 the results of their estimation should so greatly vary ; and it was this fact which 

 led me to look more closely into the matter, and to inquire whether, after all, 

 the seeming imity of standard was not more a fancied than a real one. Structure 

 may be considered from many points of view: first, with reference to the plan 

 adopted in framing it ; secondly, with reference to the work to be done by it, and 

 to the ways and means employed in building it up ; thirdly, with reference to the 

 degrees of perfection or complication it exhibits, which may differ greatly, even 

 though tlie plan ])e the same, and the ways and means employed in carrying out 

 such a plan should not differ in the least ; fourthly, with reference to the form 

 of the whole structure and its parts, which bears no necessary relation, at all events 

 no very close relation, to the degree of perfection of the structure, nor to the 

 manner in which its plan is executed, nor to the plan itself, as a comparison 

 between Bats and Birds, between Whales and Fishes, or between Ilolothurians and 

 Worms, may ea.sily show ; fifthly and lastly, vrith reference to its last finish, to 

 tlie execution of the details in the individual parts. 



It would not be difficult to show, that the differences which exist among 

 naturalists in their liuiitation of classes have arisen from an indiscriminate con- 

 sideration of the structure of animals, in all these diflerent points of view, and an 



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