146 PHILOSQPHY OF ZOOLOGY. 



was not unknown to the author, that the white colour was 

 not common to all species of the class. The institution of 

 the third class was not here necessary, as the animals which 

 it included obviously appeared deserving of holding the 

 rank of an order only in the second class. The system of 

 CUVIER, though free from the error of assigning characters 

 to classes which do not apply to all the orders included 

 under it, (although in the subordinate groups this error is 

 of frequent occurrence), is still faulty in the employment of 

 modifications of positive characters, along with positive and 

 negative ones, in the construction of classes of the first degree. 

 In the application of the mixed method, by the employ- 

 ment of positive and negative characters, it will frequent- 

 ly be found difficult to obtain a division into equal num- 

 bers of species ; one division probably including twice as 

 many as the other. This inequality, however, in many 

 cases, proceeds from the attention bestowed on the examina- 

 tion of one tribe in preference to another, and perhaps from 

 the extinction of some species which belonged to the pre- 

 sent order of animals. Yet, whether a natural, mixed, or 

 artificial method be employed, there is no numerical equa- 

 lity among the species included under each of the divisions. 

 The various subdivisions which occur in the mixed, or 

 indeed in any other method hitherto proposed, can never 

 be regarded as co-ordinate, although the terms by which 

 they are designated intimate an equality of rank. The 

 same characters which form an order or genus among birds, 

 are not employed, or rather do not exist, in fishes ; the 

 orders and genera of which are founded on characters de- 

 rived from other sources. There never, therefore, can ex- 

 ist that relative subordination of groups, in the different di- 

 visions, which many naturalists seem to acknowledge, by 

 the anxiety they display to have the same terms placed in 

 the same rank of succession, even where there is no neces- 

 sity for their employment. Zoologists have not yet ventu- 



