CLASSIFICATION AND THE PREDICABLES. 



83 



differenc3 ; or, to state the same pro- 

 position in other words, the Differentia 

 is that which must be added to the 

 connotation of the genus, to complete 

 the connotation of the species. 



The word man, for instance, exclu- 

 sively of what it connotes in connnon 

 with animal, also connotes rationality, 

 and at least some approximation to 

 that external form which we all know, 

 but which, as we have no name for it 

 considered in itself, we are content to 

 call the human. The Differentia, or 

 specific difference, therefore, of man, 

 as referred to the genus animal, is 

 that outward form and the possession 

 of reason. The Aristotelians said, the 

 possession of reason, without the out- 

 ward form. But if they adhered to 

 this, they would have been obliged to 

 call the Houyhnhnms men. The 

 question never arose, and they were 

 never called upon to decide how such 

 a case would have affected their notion 

 of essentiality. However this may be, 

 they were satisfied with taking such 

 a portion of the differentia as sufficed 

 to distinguish the species from all 

 other existing things, though by so 

 doing they might not exhaust the 

 connotation of the name. 



§ 6. And here, to prevent the no- 

 tion of differentia from being restric- 

 ted within too narrow limits, it is 

 necessary to remark, that a species, 

 even as referred to the same genus, 

 will not alwa)^ have the same dif- 

 ferentia, but a different one, accord- 

 ing to the principle and purpose which 

 preside over the particular classifica- 

 tion. For example, a naturalist sur- 

 veys the various kinds of animals, and 

 looks out for the classification of them 

 most in accordance with the order in 

 which, for zoological purposes, he con- 

 siders it desirable that we should 

 think of them. With this view he 

 finds it advisable that one of his funda- 

 mental 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 carnivorous 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 fa- 

 miliarly and sixmtaneously referred; 

 nor should we ever think of assigning 

 to them so prominent a position in 

 our arrangement of the animal king- 

 dom, unless for a preconceived pur- 

 pose 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 charac- 

 terises : 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 deriva- 

 tive properties were of primary im- 

 portance for the purposes of the na- 

 turalist, 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 arrange- 

 ment of objects run in lines not coin- 

 ciding 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 ; 

 d foi'tiori must we be warranted, 

 when our genera and species are real 

 genera and species, in marking the 

 distinction between them by those of 

 their properties which considerations 

 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 man. Suppose, however, that, 



