CLASSIFICATION 41 



a kind of centre for the vegetative functions, as the brain 

 and the spinal cord are for the animal functions." 1 These 

 two organ-systems vary in harmony, and their characters 

 must form the basis for the delimitation of the great 

 groups. Judged by this standard there are four principal 

 types of form,' 2 of which all the others are but modifica- 

 tions. These four types are Vertebrates, Molluscs, Articu- 

 lates, and Radiates. The first three have bilateral, the 

 last has radial symmetry. Vertebrates and Molluscs have 

 blood-vessels, but Articulates show a functional transition 

 from the blood-vessel to the tracheal system. Radiates 

 approach the homogeneity of plants ; they appear to lack 

 a distinct nervous system and sense organs, and the lowest 

 of them show only a homogeneous pulp which is mobile and 

 sensitive. All four classes are principally distinguished from 

 one another by the broad structural relations of their 

 neuromuscular system, of the organs of the animal functions. 

 Vertebrates have a spinal cord and brain, an internal skeleton 

 built on a definite plan, with an axis and appendages ; in 

 Molluscs the muscles are attached to the skin and the shell, 

 and the nervous system consists of separate masses ; Articu- 

 lates have a hard external skeleton and jointed limbs, and 

 their nervous system consists of two long ventral cords ; 

 Radiates have ill-defined nervous and muscular systems, and 

 in their lowest forms possess the animal functions without 

 the animal organs. 



This well-rounded classification of animal forms is in 

 a sense the crown of Cuvier's work, for the principle of the 

 subordination of characters, in the interpretation which he 

 gives to it, is a direct application of his principle of functional 

 correlation. Each of the great groups is built upon one 

 plan. The idea of the unity of plan has become for Cuvier 

 a commonplace of his thought, and it is tacitly recognised 

 in all his anatomical work. But he never takes it as a hard- 

 and-fast principle which must at all costs be imposed upon 

 the facts. 



Cuvier has become known as the greatest champion of 

 the fixity of species, but it is not often recognised that his 



1 Rcgnc A nimal, p. 55. 



2 First propounded by Cuvier in 1812, Ann. Mus.d'Hist, Nat., xix. 



