T. HILAIRE, OKEN, TOX BAER. 137 



Natural Philosophy). In France, Etienne Geoffrey St. Hilaire pre- 

 eminently defended the idea, which had been already expressed by 

 Buftbn, of the unity of the plan of animal structure, according to 

 which the animal kingdom consisted of an unbroken gradation of 

 animals. Convinced that nature always worked with the same 

 materials, he put forward his theory of analogies, according to which 

 the same parts, though differing in their form and the degree of their 

 development, should be found in all animals ; and, further, his theory 

 of connections (principe des connexions), according to which the same 

 parts always appear in the same mutual position. A third funda- 

 mental principle was that of the equivalence of organs, an increase in 

 the size of one organ being accompanied by the diminution of another 

 organ. The application of this principle had important results, and 

 led to the scientific foundation of Teratology. His generalizations 

 were, however, in the main hasty, in that they were founded on 

 facts taken only from the Vertebrates ; and if applied outside that 

 group must lead to many rash conclusions, e.g., that Insects are 

 Vertebrates turned on to their backs. 



In Germany, Goethe and the natural philosophers Oken and 

 Schelling pronounced in favour of the unity of animal organization, 

 but it must be confessed without taking account in a comprehensive 

 manner of the actual facts. 



The result of this controversy which in France was carried on 

 with considerable vehemence was, that Cuvier's view was victorious, 

 and his principles met with the more undivided assent since it 

 appeared that they were confirmed by C. E. v. Baer's embryological 

 work. Many gaps and errors were certainly discovered by later 

 investigators in Cuvier's classification, and in detail it was much 

 changed, but the establishment of his animal types as the chief 

 groups of the system was retained, and was supported by the 

 results of the developing Science of Embryology. 



The most essential of the modifications which it has become neces- 

 sary to make in Cuvier's system relate chiefly to the increase in the 

 number of types. The Infusoria were some time ago removed from 

 the Radiata, and as Protozoa arranged by the side of the four other 

 groups. Lately the number of groups has been increased by the 

 division of the Radiata into Ccelenterata and Echinodermata, and of 

 the Articulata into Arthropoda and Vcrrnes, and of the Mollusca 

 into three groups. 



In our times, however, Cuvier's view has experienced an essential 

 modification in favour of the Natural Philosophers, and the idea of 



