PARTS OF ANIMALS, I. iii.-iv. 



that, it is necessary that there should be many 

 differentiae, not under one hne of division. And yet 

 there cannot be more than one differentia for the same 

 thing under one Hne of dichotomy : one hne must end 

 in one differentia. So it is impossible for those who 

 follow the method of twofold division to arrive at any 

 of the particular animals.'* 



IV. Some may find it puzzling that general usage 

 has not combined the water-animals and the feathered 

 animals into one higher group, and adopted one name 

 to cover both, seeing that in fact these two groups 

 have certain features in common. The answer is that 

 in spite of this the present grouping is the right one ; 

 because while groups that differ only " by excess " 

 (that is, " by the more and less " *) are placed 

 together in one group, those which differ so much 

 that their characteristics can merely be called ana- 

 logous are placed in separate groups. As an illus- 

 tration : {a) one bird differs from another bird " by 

 the more," or " by excess " : one bird's feathers 

 are long, another's are short ; whereas (6) the 

 difference betΛveen a Bird and a Fish is greater, 

 and their correspondence is only by analogy : a fish 

 has no feathers at all, but scales, Avhich correspond 

 to them. It is not easy to do this in all cases, for 

 the corresponding analogous parts of most groups of 

 animals are identical. 



Now since the ultimate species are " real things," ^ Method. 

 while within them are individuals which do not differ 

 in species (as e.g. Socrates and Coriscus),'* we shall 

 have to choose (as I have pointed out) * between 



' Lit. " substances." 



' i.e. within the species " man." 



• Above, at 639 a, b, etc. 



93 



