ii. I. 19 



BOOK II. 



(Ch. I.) The nature and the number of the parts of which 

 animals are severally composed are matters which have already 

 been set forth in detail in the book of Researches about Animals.^ 

 We have now to inquire what are the causes that in each case 

 have determined this composition ; a subject reserved by us for 

 separate consideration. 



Now there are three degrees of composition ; and of these the 

 first in order, as all will allow, is composition out of what some 

 call the elements, such ^ as air, earth, water, fire. Perhaps however 

 it would be more accurate to say composition out of the elementary 

 forces ; ' nor indeed out of all of these, but out of a limited number 

 of them, as defined in previous treatises. For fluid and solid, hot 

 and cold, form the material of all composite substances ; * and all 

 other differences are secondary to these, such differences, that is, 

 as heaviness or lightness, density or rarity, roughness or smooth- 

 ness, and any other such properties of matter as there may be. 

 The second degree of composition is that by which the homo- 

 geneous parts of animals, such as bone, flesh, and the like, are 

 constituted out of the primary substances. The third and last 

 stage is the composition which forms the heterogeneous parts, such 

 as face, hand, and the rest.^ 



Now the order of actual development and the order of logical 

 existence are always the inverse of each other. For that which 

 is antecedent in the true order of nature is posterior in the order 

 of development, and that is genetically last which is logically first. 

 (That this is so is manifest by induction. For a house does not 

 exist for the sake of bricks and stones, but these materials for the 

 646 a. 



