14 THE BEGINNINGS OF COMPARATIVE ANATOMY 



second half) is largely a comparative topographical anatomy ; 

 in it, for instance, he describes the venous and skeletal 

 systems. 



This distinction which Aristotle drew plays an important 

 part in all his writings on animals, particularly in his theory 

 of development. It was a distinction of immense value, and 

 is full of meaning even at the present day. No one has ever 

 given a better definition of organ than is implied in Aristotle's 

 description of the heterogeneous parts " The capacity of 

 action resides in the compound parts" (Cresswell, loc. cit., p. 

 7). The heterogeneous parts were distinguished by the 

 faculty of doing something, they were the active or executive 

 parts. The homogeneous parts were distinguished mainly 

 by physical characters (De Generatione, i., 18), but certain of 

 them had other than purely physical properties, they were 

 the organs of touch (De Partibus, ii., i, 647 a ). 



(6) In a passage in the De Generatione (ii , 3) Aristotle 

 says that the embryo is an animal before it is a particular 

 animal, that the general characters appear before the special 

 This is a foreshadowing of the essential point in von Baer's 

 law (see Chap. IX. below). 



He considers also that tissues arise before organs. The 

 homogeneous parts are anterior genetically to the hetero- 

 geneous parts and posterior to the elementary material 

 (De Parti bus, ii., i, 646^. 



(7) We meet in Aristotle an idea which later acquired 

 considerable vogue, that of the liclicllc dcs circs (or " scale of 

 beings"), that organisms, or even all objects organic or in- 

 organic, can be arranged in a single ascending series. The 

 idea is a common one; its first literary expression is found 

 perhaps in primitive creation-myths, in which inorganic things 

 arc created before organic, and plants before animals. It 

 may be recognised also in Anaximander's theory that land 

 animals arose from aquatic animals, more clearly still in 

 Anaxagoras' theory that life took its origin on this globe 

 from vegetable germs which fell to earth with the rain. 

 Anaxagoras considered animals higher in the scale than 

 plants, for while the latter participated in pleasure (when they 



