88 NAiMES AND PROPOSITIONS. 



all other existing things,' although by so doing they might not exhaiist 

 the connotation of the name. 



§ 6. And here, to prevent the notion of differentia from being 

 restricted within too naiTow limits, it is necessary to remark, that a 

 species, even as referred to the same genus, will not always have the 

 same differentia, but a different one, according to the principle and 

 pui-pose which presides over the particular classification. For ex- 

 ample, a naturalist surveys the various kinds of animals, and looks out 

 for the classification of them most in accordance with the order in 

 which, for zoological purposes, it is desirable that his ideas should 

 an'ange themselves. With this view he finds it ad\asable that one of 

 his fundamental divisions should be into warm-blooded and cold-blood- 

 ed animals ; or into animals which breathe with lungs and those which 

 breathe with gills ; or into carnivorous, and frugivorous or graminivor- 

 ous ; or into those which walk on the flat part and those which walk on 

 the extremity of the foot, a distinction on which some of Cu\'ier's fami- 

 lies 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 preconcerted purpose of scientific con- 

 venience. And to the liberty of doing this there is no limit. In the 

 examples we have given, the new 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 other- 

 wise — 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 those derivative properties were of primary 

 importance for the purposes of the naturalist, he would be warranted 

 in founding his primary division upon them. 



If, however, practical convenience is a sufficient wai'rant for making 

 the main demarcations in our aiTangement 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 distinc- 

 tion between them by those of their properties which considerations 

 of practical convenience most sti'ongly recommend. If we cut a 

 species out of a given genus — the species man, for instance, out of the 

 genus anim.al — 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 man. 

 Suppose, however, that, being naturalists, we, for the purposes of our 

 particidar study, cut out of the genus animal the same species man, 

 but with an intention that the distinction between man and all other 

 species of animal should be, not rationality, but the possession of " four 

 incisors in each jaw, tusks solitary, and erect posture." It is evident 

 that the word man, when used by us as naturalists, no longer connotes 

 rationality, but connotes the three other properties specified; for that 

 w^hich we have expressly in view when we impose a name, assuredly 

 forms part of the meaning of that name. We may, therefore, lay it 

 down as a maxim, that wherever there is a Genus, and a Species 



