CLASSIFICATION OF ANIMALS. 213 



eidos generate animals of the same eidos as themselves, and 

 that animals not of the same eidos, such as the horse 

 and the ass, generate animals of a different eidos * The 

 Hemioiwi, or half-asses of Syria, are so called, he says, 

 because of their likeness to the ass, although they are not 

 of the same eidos, for they certainly breed among them- 

 selves.! Agassiz, referring to this passage, says : — " Aristotle 

 already considers fecundity as a specific character.! 



On the whole, Aristotle's idea of an eidos was much like 

 the modern idea of a species, but there are many passages 

 in iiZ. ^. viii. and ix., which show that his eidos often had, 

 in practice, very much the same meaning as the modern 

 term genus. 



The term genos is of very wide signification, and denotes 

 a group of animals with parts of the same kind, but differing 

 in excess and deficiency ; § on the other hand, animals with 

 parts which resemble one another only by analogy belong 

 to different gene.W 



In accordance with the principles thus laid down, Aris- 

 totle forms a genos of Ornithes, and another of Ichthyes, 

 both of the first magnitude, i.e., containing blood ; he also 

 forms a third genos, the Selache, comprising fishes in which 

 cartilage takes the place of bone. He found difficulties in 

 the further application of this method, for he says that it is 

 not easy to arrange all animals in this w^ay, because so many 

 of them present the same analogous structure. II In the more 

 difficult cases, then, he has recourse to other means, and bases 

 the formation of his gene — the Kete or Ketode, Malakia, 

 Malakostraka, Ostrahoderma, and Entoma, and the small 

 group of the Lophoura — on the existence of certain struc- 

 tural features. 



The above are the best-defined of Aristotle's gene, and 

 constitute the best proof that he attempted to form a 

 systematic classification. He says that his Ornithes, 

 Ichthyes, and Ketode are his most important classes {gene 

 megista), because they include animals with blood, and that 

 the Malakia, Malakostraka, Ostrakoderma, and Entoma, 

 animals without blood, are important classes {gene megala), 

 there being no important gene other than these.** In 

 various other passages, he speaks of a genos of snakes, one 



* G. A. ii. c. 8, 7476, and 748a. f H. A. i. c. G, s. 3. 



\ Op. cit. p. 801. § fl". il. i. c. 1, s. 2 ; P. A. i. c. 4, 644a, 



]| H. A. ii. c. 1, s. 1 ; P. A. \. c. 4, 644rt. U P. A. i. c. 4, 644a, 



** H. A. i. c, 6, s. 1. 



