BRANCHES OF THE ANIMAL KINGDOM. 219 



upon material relations. The lesser divisions which suc- 

 ceed next are founded upon special qualifications of the 

 plan, and differ one from the other in the character of 

 these qualifications. Should it be found that the features 

 in the animal kingdom, which, next to the plan of struc- 

 ture, extend over the 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. Ex- 

 perience, 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 extensive divisions (the 

 classes) than those which are based upon the complica- 

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

 natural only so far as it expresses real relations observed 

 in nature, it follows, therefore, that classes take the second 

 position in a system, immediately under the branches. 

 We shall see below that orders follow next, as they natu- 

 rally constitute groups that are more comprehensive 

 than families, and that we are not at liberty to invert 

 their respective position 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 king- 

 dom is constructed upon four different plans of structure, 

 classes were the highest groups acknowledged in the sys- 

 tems of Zoology, and naturaljgts at a very early period under- 

 stood upon what this kind of division should be founded, in 



