212 CLASSIFICATION OF ANIMALS. 



Singly or in association these terms occur in not less 

 than three hundred passages in Aristotle's zoological works, 

 and also in many other passages, chiefly in his Organon. 

 Most of these passages merely give examples of the use of 

 the terms, but some explain their meanings, and the fol- 

 lowing discussion is based on several of these explanatory 

 passages. 



Particular animals, or individuals, such as Socrates or 

 Coriscos, are essences, or actual existences, exhibiting dif- 

 ferences which distinguish one from another.* These 

 essences have certain features in common, and a group 

 may be formed of such essences. Such a group may be 

 defined by means of the term eidos, provided the common 

 features more closely indicate the nature of the essences 

 included in the group, or by means of the term genos, if the 

 common features indicate the nature of the essences less 

 closely. Thus, the nature of Socrates or Coriscos is more 

 closely defined by the name Man than by the name animal, 

 which, in this case, represent eidos and genos respectively.! 



Animals of which the parts, internal as well as external, 

 are the same, belong to the same eidos, I e.g., Man con- 

 stitutes one eidos, and the horse another, § and Aristotle also 

 ' states that the parts necessary to an animal, e.g., the parts 

 for receiving and digesting food, the locomotory parts, and 

 some of the sense organs, should be the same in animals 

 belonging to the same eidos. \\ Differences in essential, and 

 not accidental, features should alone be considered, and, 

 among examples of accidental features or qualities, he 

 mentions the whiteness of snow and the equivalence of the 

 angles of a plane triangle to two right angles. IT Evidently, 

 although he says that the parts of animals belonging to the 

 same eidos should be the same, he does not mean that these 

 parts should be alike in all respects. He often refers, in 

 fact, to differences in colour, shape, and relative sizes of 

 parts, such as, for instance, the eyes, ears, and locomotory 

 and other parts, in different individuals of the same eidos. 



The capability of generating fertile offspring has often 

 been considered to be important in defining a species. 

 Aristotle also considered it to be important in connection 

 with his views on eidos. He says that animals of the same 



■''• P. A. i. c. 4, Giia ; De Long, et Brev. Vitce, c. 1 ; Categ. c. 3. 

 i Categ. c. 3. + H. A. i. c. 1, s. 2. 



§ Topica, i. c. 5, s. 2. || Politica, iv. c. 3, ss. 9 and 10. 



i\ Topica, iv. c. 1, ss. 1-3 ; P. A. i. c. 3, G43rt. 



