142 NAMES AND PROPOSITIONS. 



zoological purposes, he considers it desirable that we should 

 think of them. With this view he finds it advisable that 

 one of his fundamental divisions should be into warm-blooded 

 and cold-blooded animals ; or into animals which breathe 

 with lungs and those which breathe with gills ; or into car- 

 nivorous, and frugivorous or graminivorous ; or into those 

 which walk on the flat part and those which walk on the 

 extremity of the foot, a distinction on which two of Cuvier's 

 families are founded. In doing this, the naturalist creates as 

 many new classes ; which are by no means those to which the 

 individual animal is familiarly and spontaneously referred ; 

 nor should we ever think of assigning to them so prominent 

 a position in our arrangement of the animal kingdom, unless 

 for a preconceived purpose of scientific convenience. And to 

 the liberty of doing this there is no limit. In the examples 

 we have given, most of the classes are real Kinds, since each 

 of the peculiarities is an index to a multitude of properties 

 belonging to the class which it characterizes : but even if the 

 case were otherwise if the other properties of those classes 

 could all be derived, by any process known to us, from the 

 one peculiarity on which the class is founded even then, if 

 these derivative properties were of primary importance for the 

 purposes of the naturalist, he would be warranted in founding 

 his primary divisions on them. 



If, however, practical convenience is a sufficient warrant 

 for making the main demarcations in our arrangement of 

 objects run in lines not coinciding with any distinction of 

 Kind, and so creating genera and species in the popular 

 sense which are not genera or species in the rigorous sense 

 at all, a fortiori must we be warranted, when our genera 

 and species are real genera and species, in marking the dis- 

 tinction between them by those of their properties which con- 

 siderations of practical convenience most strongly recommend. 

 If we cut a species out of a given genus the species man, 

 for instance, out of the genus animal with an intention 

 on our part that the peculiarity by which we are to be 

 guided in the application of the name man should be 

 rationality, then rationality is the differentia of the species 



