X INTRODUCTION. 



places as though Nature worked under some kind of restraint, the 

 obligation of which is never clearly defined, whether, that is, it is self- 

 imposed or dictated from without. This restraint consists in the 

 existence of certain definite types or patterns, in more or less close 

 accordance with which each organism must be made. Nature, for 

 instance, can fashion this or that bird to fit the exact mode of life 

 / which is pre-ordained for its species ; but in so fashioning it she must 

 I not transgress certain limits. Each and all birds, through all their 

 varieties, must present in form and composition ^ the essential characters 

 of the ideal bird which constitutes the avian type. If any part in this 

 type be useless to the special animal created, nay, even if its presence 

 be actually prejudicial,^ yet it must still be there in some shape or other, 

 " by way of token " (arjfjLeiov ')(apiv)- The most that Nature in such a 

 case can do is to reduce its size ; and, inasmuch as the ultimate com- 

 position of all the animals in a given class is precisely, or almost pre- 

 cisely,' alike, she can only do this by diverting the material which should 

 have gone to the full formation of the useless or prejudicial part into 

 some other organ where it may be of use, or at any rate not equally 

 injurious. So that, in fact, the organisation of an individual species 

 of animals is not always the best conceivable, but the best of which 

 the essential type of the class to which that species belongs admits.^ 

 Aristotle, as I have said, never expresses himself clearly on this matter. 

 Did Nature herself make these types, and adhere to them in her after- 

 work with the obstinacy of a prejudiced inventor ; or, was there some 

 external necessity coercing her, and driving her against her own better 

 judgment to make her products in part futile ? I take it that Aristotle 

 was not himself clear as to his own views on the matter ; that his 

 opponents, or his own mind, had pointed out the impossibility of 

 ^ preconciling the existence of r udimentarv orga ns with the strictly teleo- 



yl logical position, "^and that he met the difficulty with a phrase, " by way 

 of token," leaving it really unexplained. 



The giant share which Aristotle allots to final causes, and the almost 

 complete exclusion of necessity from consideration, make the main 

 portion of his treatise to consist of little more than an attempt to assign 

 to each part of the animal body a definite use ; so that the work 

 becomes rather one on the functions of parts than on the causes of 

 their existence, and might almost have been styled, as was Galen's 

 later work, a treatise "De Usu partium." Very possibly it was on this 

 account that the designation by which Aristotle himself* refers to the 

 work, " On the Causes of the Parts of Animals," was in time superseded 

 by the vaguer title " De Partibus," which it has ever since borne. 



» D. r. iv. 12, 28. 2 D. p^ iii_ 2, 5. 3 De incessu. 2, i, and 8, r. * D. G. v. 3, 6. 



