TYPES IN THE ANIMAL KINGDOM. 1 1 9 



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 

 gi-eater 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 Linnseus' first four classes — mammals, birds, 

 amphibious animals, and fishes. The second type is formed 

 by the articulated animals (Articulata), containing Linnseus' 

 insects, consequently the six-legged insects, and also the 



