CLASSIFICATION. 373 



\Ve have, then, as Mr. Bentham says, three bifid divi- 

 sions, thus represented : 



Vertebrata 



Birds Reptiles 



It is however quite evident that according to the laws 

 of thought even this arrangement is incomplete. The 

 subclass mammifera must either have wings or be deprived 

 of them ; we must subdivide this class, or assume that 

 none of the mainmifera have wings, which is, as a matter 

 of fact, the case, the wings of bats not being true wings in 

 the meaning of the term as applied to birds. Fish, again, 

 ought to be considered with regard to the possession of 

 rnammaB and wings ; and in leaving them undivided we 

 really imply that they never have mamma? nor wings, the 

 wings of the flying-fish, again, being no exception. If 

 we resort to the use of our letters and define them as 

 follows 



A = vertebrata, 



B = having lungs, 



O O 7 



C = having mammae, 

 D = having wings, 



then there are four existent classes of vertebrata which 

 appear to be thus described 



ABC 

 ABcD 

 KEcd 

 A6. 



But in reality the combinations are implied to be 



