148 ESSAY ON CLASSIFICATION 



largest divisions are those which determine their rank or respective 

 standing, it would appear natural to consider the orders as the second 

 most important category in the organization of animals. Experience, 

 however, shows that this is not the case; that the manner in which the 

 plan of structure is executed leads to the distinction of more exten- 

 sive divisions (the classes) than those which are based upon the com- 

 plication of structure (the orders). As a classification can be natural 

 only as far as it expresses real relations observed in nature, it follows 

 therefore that classes take the second position in a system immedi- 

 ately under the branches. We shall see below that orders follow next, 

 as they constitute naturally groups that are more comprehensive than 

 families, and that we are not at liberty to invert their respective posi- 

 tion, nor to transfer the name of one of these divisions to the other at 

 our own pleasure, as so many naturalists are constantly doing.'^ 



SECTION II 



CLASSES OF ANIMALS 



Before Cuvier had shown that the whole animal kingdom is con- 

 structed upon four different plans of structure, classes were the high- 

 est groups acknowledged in the systems of Zoology, and naturalists 

 very early understood 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 ex- 

 pounder 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, 



'' [In the preceding discussion Agassiz's insistence on the ultimate rationaUty of nature 

 as demonstrated by an underlying "plan" is exemplified. This traditional and idealis- 

 tic viewpoint held that species and genera enjoyed a "real" existence in that they 

 symbolized that fundamental reality, the categories and forms of divine thought. The 

 essential task of the naturalist as empiricist was to describe and analyze the individual 

 representations in nature of such higher forms.] 



