CLASSIFICATION. 389 



Mr. Bentham also gives a bifurcate arrangement of 

 animals after the method proposed by Dumeril in his 

 * Zoologie Anal ytique/ this naturalist being distinguished 

 by his clear perception of the logical importance of the 

 method. 



A more recent binary classification of the animal king- 

 dom as regards the larger classes may be found in Pro- 

 fessor Eeay Greene's ' Manual of the Ccelenterata/ p. 18. 



Does Abstraction imply Generalization ? 



Before we can acquire a sound comprehension of the 

 subject of classification we must answer a very difficult 

 question, namely, whether logical abstraction does or does 

 not always imply generalization. It comes to exactly the 

 same thing if we ask whether a species may be coexten- 

 sive with its genus, or whether, on the other hand, the 

 genus must contain more than the species. To abstract 

 logically is, as we have seen (vol. i. p. 33), to overlook or 

 withdraw our notice from some point of difference. When- 

 ever we form a class we abstract, for the time being, the 

 differences of the objects so united in respect of some 

 common quality. If, for instance, we class together a 

 great number of objects as dwelling-houses, we overlook 

 or abstract the fact that some dwelling-houses are con- 

 structed of stone, others of brick, wood, iron, &c. Very 

 often at least the abstraction of a circumstance increases 

 the number of objects included under a class according to 

 the law of the inverse relation of the quantities of exten- 

 sion and intension (vol. i. p. 32). Dwelling-house is a 

 wider term than brick dwelling-house. House, or building, 

 is more general still than dwelling-house. But the ques- 

 tion before us is, whether abstraction always increases the 

 number of objects included in a class, which amounts to 

 asking whether the law of the inverse relation of logical 

 quantities is always true. The interest of the question 



