CLASSIFICATION OF ANIMALS. 211 



but others have an ' analogon,' which has the same power, 

 or function, as blood."* 



In Aristotle's idea of analogy, similarity of functions of 

 the analogous parts is certainly included, but there is a 

 passage, difficult to understand, which appears to introduce 

 another factor, that of correspondence in the positions of 

 certain parts of different animals; for, in P. A. iv. c. 5, 

 6816, he says that it is evident, from the position of the 

 so-called mytis of a cephalopod, that this part is the analogon 

 of the heart of other animals. This is proved, he adds, by 

 the sweetness of its contained liquid, which is of the nature of 

 blood. It is probable that the mytis, to which he refers, 

 was the liver. He also says, in H. A. i. c. 6, s. 2, that a 

 scute corresponds in position with a scale, and, in numerous 

 passages, he refers to a relationship between such parts as the 

 arms of Man, the forelegs of quadrupeds, the wings of birds, 

 and the pectoral fins of fishes, which are now known to be 

 homologous parts. Agassiz says: — "Though Aristotle already 

 knew that the scales of fishes correspond to the feathers of 

 birds, it is but recently that anatomists have discovered the 

 close correspondence which exists between all the parts of all 

 animals belonging to the same type, however different they 

 may appear at first sight. Not only is the wing of the bird 

 identical in its structure with the arm of man or the fore- 

 leg of a quadruped, but it agrees quite as closely with the 

 fin of the whale or the pectoral fin of the fish ; . . . But 

 this correspondence is not limited to the skeleton ; every 

 other system of organs exhibits in these animals the same 

 relations, the same identity in plan and structure, whatever, 

 be the differences in the form of the parts, in their number, 

 and even in their functions."! 



It cannot be decided to what extent, if any, Aristotle 

 was thinking of the plan of structure of the parts, when he 

 compared them, but it is clear that he was referring chiefly 

 to their functions, positions, and mere external resemblances. 



Two very important terms of classification, employed by 

 Aristotle, may now be considered, viz., genos and eidos. 

 These are often translated as " genus " and " species " re- 

 spectively. In many cases, eidos may be translated fairly 

 well in this way, but genos usually signifies a class, an order, 

 or a family. 



* P. A. I c. 5, 6456. 

 f An Essay on Classification, 1859, pp. 25, 20. 



