142 PHILOSOPHY OF ZOOLOGY. 



of the animal Mngdom. These are few in number, and 

 nave already come under our consideration *. If we lay 

 aside these, and attentively examine the remaining charac- 

 ters, we shall find some of them common to extensive 

 groups, while others make their appearance in a small num- 

 ber of species. If we take a hundred species, for example, 

 it will be practicable, in many cases, to discover characters 

 which are common to nearlyfifty of these, but which are either 

 absent or incorporated with other characters in the remain- 

 der. We thus obtain two classes of fifty each, the one dis- 

 tinguished by a positive, the other by a negative mark. 

 If we procure an animal, and wish to determine to which of 

 these hundred species it belongs, we first determine its 

 class, from its positive or negative characters, and we are 

 thus saved the trouble of comparing it with the fifty species 

 of the class to which it does not belong. If we subject 

 each of these classes to a similar examination as that which 

 led to their discovery, we may be able to effect a similai* 

 twofold division, and thus facilitate, in an equal degree, 

 the labour of the comparison. If we proceed in this man- 

 ner, halving every successive division, we shall ultimately 

 reach that limited number of species with which the indi- 

 vidual we possess can be readily compared. 



It must appear obvious, that, by this mode of classifica- 

 tion, a great deal of tedious and useless repetitions will 

 be avoided in the descriptions of species, and the enu- 

 meration of the marks of genera. The characters of the 

 class will comprehend all the properties common to its 

 orders, genera, and species. The characters of the order 

 will include all the properties common to its genera and 

 species. The characters of the genus will include all 

 the properties common to the species; and in the de- 

 scription of a species, it will only be necessary to enumerate 



* Vol. i. chap. 3. 



