TYPES IN THE ANIMAL KINGDOM. II9 
which possessed an entirely peculiar type or structure (com- 
pare above, vol. i. p. 53). In each of these main divisions 
there is a tree-shaped and branching gradation from most 
simple and imperfect forms to those which are exceedingly 
composite and highly developed. The degree of development 
within each type is quite independent of the peculiar plan 
of structure, which forms the basis of the type and gives it 
a special characteristic. The “type” is determined by the 
peculiar relations in position of the most important parts of 
the body, and the manner in which the organs are connected. 
The degree of development, however, is dependent upon the 
greater or less division of labour among organs, and on the 
differentiation of the plastids and organs. This extremely 
important and fruitful idea was established by Bar, who 
relied more distinctly and thoroughly upon the history of 
individual development than did Cuvier. Cuvier based 
his argument upon the results of comparative anatomy. 
But neither of them recognized the true cause of the re- 
markable relationships pointed out by them, which is first 
revealed to us by the Theory of Descent. It shows us that 
the common type or plan of structure is determined by in- 
heritance, and the degree of development or differentiation 
by adaptation. (Gen. Morph. ii. 10). 
Both Bar and Cuvier distinguished four different types in 
the animal kingdom, and divided it accordingly into four 
great main divisions (branches or circles). The first of these 
is formed by the vertebrate animals (Vertebrata), and 
comprises Linnzeus’ first four classes—mammals, birds, 
amphibious animals, and fishes. The second type is formed 
by the articulated animals (Articulata), containing Linnzeus’ 
insects, consequently the six-legged insects, and also the 
