516 HISTORY OF PHYSIOLOGY. 



in his way of applying it, we look upon as a material 

 advance in physiological knowledge, and therefore 

 give to it a distinct place in our history. " Zoology 

 has," he says 20 , in the outset of his Regne Animal, 

 " a principle of reasoning which is peculiar to it, and 

 which it employs with advantage on many occasions : 

 this is the principle of the conditions of existence, 

 vulgarly called the principle of final causes. As 

 nothing can exist if it do not combine all the con- 

 ditions which render its existence possible, the dif- 

 ferent parts of each being must be co-ordinated in 

 such a manner as to render the total being possible, 

 not only in itself, but in its relations to those which 

 surround it; and the analysis of these conditions 

 often leads to general laws, as clearly demonstrated 

 as those which result from calculation or from ex- 

 perience." 



This is the enunciation of his leading principle 

 in general terms. To our ascribing it to him, some 

 may object, on the ground of its being self-evident 

 in its nature 21 , and having been very anciently 

 applied. But to this we reply, that the principle 

 must be considered as a real discovery, in the hands 

 of him who first shows how to make it an instru- 

 ment of other discoveries. It is true in other cases 

 as well as in this, that some vague apprehension of 

 true general principles, such as a priori considera- 

 tions can supply, has long preceded the knowledge 

 of them as real and verified laws. In such a way 



20 Regne An. p. 6. 21 Swainson, Study of Nat. Hist. p. 85. 



