PARTS OF ANIMALS, I. iii. 



differentia will belong to two species. And if so, 

 it is clear that a privative cannot be a valid 

 differentia.^ 



(d) Now assuming that each species is indivisible : if 

 each differentia also is indivisible, and none is common 

 to more species than one, then the number of differ- 

 efitiae will be equal to the number of species. (Suppos- 

 ing it were possible to have a differentia which though 

 indivisible was common ; clearly, in that case, animals 

 which differed in species would be in the same division 

 in virtue of that common differentia. Therefore, if the 

 differentiae under which the indivisible and ultimate 

 species fall are to be proper and private to each 

 one, it is necessary that no differentia be common ; 

 otherwise, species which are actually different will 

 come under one and the selfsame differentia.^ And 

 we may not place one and the same indivisible 

 species under two or three of the lines of differentia- 

 tion given by the divisions ; nor may we include 

 different species under one and the same line of 

 differentiation. Yet each species must be placed 

 under the lines of differentiation available. It is 

 evident from this that it is impossible to arrive at the 

 indivisible species either of animals or of any other 

 group by the method of twofold division as these 

 people practise it, for even on their showing the 

 number of ultimate differentiae must of necessity be 

 equal to the total number of indivisible species of 

 animals. Thus, suppose M^e have some particular 

 group of creatures whose prime differentiae are 

 " pale " and " not pale " ; by that method these 

 two will each give two other differentiae, and so 

 forth, until in the end the indivisible differentiae are 

 reached : these last ones will be either four in 



85 



