500 HISTORY OF PHYSIOLOGY. 



the most important discoveries which have been 

 made. Of the views of these two schools we must 

 endeavour to give some account. 



The disciples of the former of the two schools 

 express their tenets by the phrases unity of plan, 

 unity of composition ; and the more detailed deve- 

 lopement of these doctrines has been termed the 

 Theory of Analogues, by Geoffrey Saint-Hilaire, who 

 claims this theory as his own creation. According 

 to this theory, -the structure and functions of animals 

 are to be studied by the guidance of their analogy 

 only; our attention is to be turned, not to the fit- 

 ness of the organization for any end of life or action, 

 but to its resemblance to other organizations by 

 which it is gradually derived from the original type. 



According to the rival view of this subject, we 

 must not assume, and cannot establish, that the plan 

 of all animals is the same, or their composition simi- 

 lar. The existence of a single and universal system of 

 analogies in the construction of all animals is entirely 

 unproved, and therefore cannot be made our guide 

 in the study of their properties. On the other hand, 

 the plan of the animal, the purpose of its organiza- 

 tion in the support of its life, the necessity of the 

 functions to its existence, are truths which are irre- 

 sistibly apparent, and which may therefore be safely 

 taken as the bases of our reasonings. This view has 

 been put forwards as the doctrine of the conditions 

 of existence : it may also be described as the prin- 

 ciple of a purpose in organization ; the structure 



