FOREWORD 



unrecognized. This Avas the doctrine of internal 

 finality which was generally accepted until DarAvin 

 elaborated his theory of Natural Selection. The 

 wider doctrine of external finality, according to 

 which living beings are ordered in regard to one 

 another, never gained acceptance among scientific 

 philosophers, and the only indication that Aristotle 

 ever adopted it is furnished by a passage in \vhich 

 he suggests that the mouth in Selachians is placed 

 on the under surface so as to allo\v their prey to 

 escape while the fish are turning on their backs 

 before taking their food ; but even this he qualified 

 by the suggestion that the arrangement served a 

 useful end for the fishes in question by preventing 

 them from indulging in the harmful habit of gluttony. 

 The De partibus animalium opens with an intro- 

 duction devoted to general considerations. This is 

 followed by a discussion of the three degrees of 

 composition, the first degree being composition of 

 physical substances, the second degree, of homo- 

 geneous parts or tissues, and the third, of hetero- 

 geneous parts or organs. The tissues referred to are 

 blood, fat, marrow, brain, flesh, and bone. After 

 describing these, the organs are dealt Λvith, and a 

 consideration of their respective functions, first in 

 sanguineous animals (i.e. in Vertebrates), and secondly 

 in bloodless animals (i.e. Invertebrates), occupies the 

 remainder of the book. The account given of the 

 physiology of the blood is especially interesting, and 

 it is noteworthy that Aristotle understood something 

 of the nature of the process of absorption whereby 

 the food becomes converted into nutriment which is 

 carried by the blood to all parts of the body. He 

 supposed, however, that the matter derived from the 



