52 THE HISTOEY OF CEEATION. 



rate and easily discoverable signs in the number, size, place, 

 and form of individual organic parts of the body, Cuvier 

 penetrated much more deeply into the essence of organiza- 

 tion. He demonstrated great and wide differences in the 

 inner structure of animals, as the real foundation of a 

 scientific knowledge and classification of them. He dis- 

 tinguished natural families in the classes of animals, and 

 established his natural system of the animal kingdom on 

 their comparative anatomy. 



The progress from Linnseus' artificial system to Cuvier's 

 natural system was exceedingly important. Linnaeus had 

 arranged all animals in a single series, which he divided 

 into six classes, two classes of Invertebrate, and four classes 

 of Vertebrate animals. He distinguished these artificially, 

 according to the nature of their blood and heart. Cuvier, 

 on the other hand, showed that in the animal kingdom there 

 were four great natural divisions to be distinguished, which 

 he termed Principal Forms, or General Plans, or Branches 

 of the animal kingdom (Embranchments), namely — 1. The 

 Vertebrate animals ( Vertebra ta) ; 2. The Articulate animals 

 (Articulata) ; 3. The Molluscous animals (Mollusca) ; and 4. 

 The Radiate animals (Radiata). He further demonstrated 

 that in each of these four branches a peculiar plan of struc- 

 ture or type was discernible, distinguishing each branch 

 from the three others. In the Vertebrate animals it is dis- 

 tinctly expressed by the form of the skeleton, or bony 

 framework, as also by the structure and position of the 

 dorsal nerve-chord, apart from many other peculiarities. 

 The Articulate animals are characterized by their ventral 

 nerve-chord and their dorsal heart. In Molluscs the sack- 

 shaped and non-articulate body is the distinguishing feature. 



