14 1. 3—1- 4- 



The impossibility of reaching the definition of any of the 

 ultimate forms by dichotomy of the larger group, as some ' 

 propose, is manifest also from the following considerations. It 

 is impossible that a single differentia, either by itself or with its 

 antecedents, shall express the whole essence of a species. (In 

 saying a single differentia by itself I mean such an isolated 

 differentia as Cloven-footed ; in saying a single differentia with 

 its antecedents I mean such a series as Footed, Two-footed, 

 Cloven-footed. The very continuity of such a series of succes- 

 sive differentiae in a division is intended to show that it is their 

 combination that expresses the character of the resulting unit, 

 or ultimate group. But one is misled by the usages of language 

 into imagining that it is merely the final term of the series, 

 Cloven-footed for instance, that constitutes the whole differentia, 

 and that the antecedent terms, Footed, Two-footed, are super- 

 fluous. Now it is evident that such a series cannot consist of 

 many terms. For if one divides and subdivides, one soon reaches 

 the final differential term ; but for all that, one* has not got to 

 the ultimate division, that is, to the species.) No single dif- 

 ferentia, I repeat, either by itself or with its antecedents, can 

 possibly express the essence of a species. Suppose, for example, 

 Man to be the animal to be defined ; the single differentia will be 

 Cloven-footed, either by itself or with its antecedents, Footed and 

 Two-footed. Now if man was nothing more than a Cloven-footed 

 animal, this single differentia would duly represent his essence. 

 But seeing that this is not the case, more differentiae than this 

 one will necessarily be required to define him ; and these cannot 

 come under one division ; for each single branch of a dichotomy 

 ends in a single differentia, and cannot possibly include several 

 differentiae belonging to one and the same animal. 



It is impossible then to reach any of the ultimate animal 

 forms by dichotomous division. 



(Ch. 4.) It deserves inquiry why a single name denoting a single 

 group was not invented by mankind, as an appellation to com- 

 prehend the two groups of Water animals and Winged animals. 

 For there are certain attributes common to the two ^ and 

 [differentiating them from] the rest of the animal kingdom. 

 However, the present nomenclature is just. Groups that only 

 differ in degree, and in the more or less of an identical element 

 that they possess, are aggregated under a single class ; groups 

 644 a. 



