2 INTRODUCTION. 



terms of which the phenomena of animal organisation 

 were to be, to a certain extent, explained. 



The three principles of Saint-Hilaire, each of which 

 contains a large element of truth, were the following : — 



■I. The Tlieory of Analogues, according to which the 

 same parts occur, in various grades of form and develop- 

 ment, in all animals. 



2. The Prinaple of Connexions (Le principe des con- 

 nexions), according to which the same parts always tend 

 to occur in similar topographical relations. 



3. The Principle of the Correlation of Organs (Le 

 principe du balancement des organes), according to which, 

 ca:tcris paribus, the bulk of the animal body remains in 

 a measure the same, and any given organ can only become 

 enlarged or reduced according as another organ becomes 

 reduced or enlarged. 



Having established these principles in his own mind 

 from the exclusive study of the Vertebrates, the thought 

 next occurred to him that probably they were capable of 

 equal application to the rest of the animal kingdom, and 

 he therefore undertook the task of identifying in the 

 Insects the typical structural peculiarities of the Verte- 

 brates. 



According to his theory he would expect to find in the 

 Insects, in some form or other, the same organs that 

 occur in the Vertebrates. At the outset he was, as his 

 successors have since been, confronted by the palpable 

 fact that, while the longitudinal nerve-cord of the Insects 

 lies next to the ventral surface of the body, the spinal 

 cord of the Vertebrates lies below the dorsal surface. 

 Accordingly he came to the conclusion which has since 

 been strongly advocated by the upholders of the so-called 

 "Annelid-theory," that the "back" and "belly" of an 



