52 THE HISTORY OF CREATION. 
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 Linnzus’ artificial system to Cuvier's 
natural system was exceedingly important. Linnzeus 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 
Jertebrate animals (Vertebrata) ; 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 Molluses the sack-. 
shaped and non-articulate body is the distinguishing feature. 
