80 Trans. Acad. Sci. of St. Louis. 



or less firmly united, but of a continuous cartilaginous case sur- 

 rounding the brain, has led to an abandonment of Goethe's and 

 Oken's theory, but it still remains of deep historical interest. 



The doctrine of homologies was much elaborated and ex- 

 tended by that master of comparative anatomy, Cuvier 

 (1769-1832), who until the time of Darwin was the most 

 commanding figure in the zoology of the century; and we 

 must now direct our attention for a moment to his influence 

 upon the development of the science. 



Owing to his vast researches in comparative anatomy, of 

 both invertebrates and vertebrates, the idea of homology of 

 parts became deeply rooted in Cuvier' s mind, and it was this 

 principle which led him to an entirely new view of the rela- 

 tionships of animals, a view which may be called the Type- 

 theory, as opposed to the scala naturae of Linnaeus and 

 others. He recognized four distinct types of structure in the 

 animal kingdom, each distinguished by a peculiar plan of 

 structure of its own; and under each branch, or embranche- 

 menl, he arranged the Linnaean groups. His classification as 

 finally elaborated and published in " Le Begne Animal" in 

 1829 is as follows: — 



First Branch. Animalia Vertebrata. 

 Class 1. Mammalia. 



" 2. Aves. 



" 3. Reptilia. 



" 4. Pisces. 

 Second Branch. Animalia Mollusca. 

 Class 1. Cephalopoda. 



" 2. Pteropoda. 



" 3. Gastropoda. 



" 4. Acephala. 



" 5. Brachiopoda. 



" 6. Cirrhopoda. 

 Third Branch. Animalia Articulata. 

 Class 1. Annelida. 



" 2. Crustacea. 



" 3. Arachnida. 



" 4. Insecta. 

 Fourth Branch. Animalia Radiata. 

 Class 1. Echinodermata. 



f< 2. Intestinal Worms. 



" 3. Acalephae. 



,f 4. Polypi. 



" 5. Infusoria. 



