Aristotle 43 



nature and contain closely allied subordinate forms, whether 

 they are groups recognized by a true instinct of mankind, 

 such as Birds and Fishes, or groups not popularly known 

 by a common appellation, but withal composed of closely 

 allied subordinate groups ; and only to deal individually with 

 the attributes of a single species, when such species man, for 

 instance, and any other such, if such there be stands apart 

 from others, and does not constitute with them a larger natural 

 group. 



' It is generally similarity in the shape of particular organs, 

 or of the whole body, that has determined the formation of the 

 larger groups. It is in virtue of such a similarity that Birds, 

 Fishes, Cephalopoda, and Testacea have been made to form 

 each a separate genus (ytvos). For within the limits of each 

 such genus, the parts do not differ in that they have no 

 nearer resemblance than that of analogy such as exists 

 between the bone of man and the spine of fish but they 

 differ merely in respect of such corporeal conditions as 

 largeness smallness, softness hardness, smoothness roughness, 

 and other similar oppositions, or, in one word, in respect of 

 degree.' 1 



The Aristotelian genus thus differs widely from the term as 

 used in modern biology. In another passage he comes nearer 

 to defining it and the analogy of parts which extends from 

 genus to genus. 



' Groups that differ only in the degree, and in the more or 

 less of an identical element that they possess are aggregated 

 together under a single genus ; groups whose attributes are not 

 identical but analogous are separated. For instance, bird differs 

 from bird by gradation, or by excess and defect ; some birds 

 have long feathers, others short ones, but all are feathered. 

 Bird and Fish are more remote and only agree in having 

 analogous organs ; for what in the bird is feather, in the 

 1 De partibus animalium, i. 4 ; 644 a 27. 



