INTRODUCTION. 33 



have now to ascertain what are the essential characters in animals, 

 on which their primary divisions are to be founded. It is evident 

 they should be those which are drawn from the animal functions, that 

 is from the sensations, and motions; for both these not only make 

 the being an animal, but in a manner establish its degree of ani- 

 mality. 



Observation confirms this position by showing that their degrees 

 of development and complication accord with those of the organs of 

 the vegetative functions. 



The heart and the organs of the circulation form a kind of centre 

 for the vegetative functions, as the brain and the trunk of the nervous 

 system do for the animal ones. Now we see these two systems 

 become imperfect and disappear together. In the lowest class of 

 animals, where the nerves cease to be visible, the fibres are no longer 

 distinct, and the organs of digestion are simple excavations in the 

 homogeneous mass of the body. In insects the vascular system even 

 disappears before the nervous one; but, in general, the dispersion of 

 the medullary masses accompanies that of the muscular agents: a 

 spinal marrow, on which the knots or ganglions represent so many 

 brains, corresponds to a body divided into numerous rings, supported 

 by pairs of limbs longitudinally distributed, &c. 



This correspondence of general forms, which results from the 

 arrangement of the organs of motion, the distribution of the nervous 

 masses, and the energy of the circulating system, should then be the 

 basis of the primary divisions of the animal kingdom. We will 

 afterwards ascertain, in each of these divisions, what characters 

 should succeed immediately to those, and form the basis of the pri- 

 mary subdivisions. 



General distribution of the Animal Kingdom into Four Great Di- 

 visions. 



If, divesting ourselves of the prejudices founded on the divisions 

 formerly admitted, we consider only the organization and nature of 

 animals, without regard to their size, utility, the greater or less 

 knowledge we have of them, and other accessary circumstances, we 

 shall find there are four principal forms, four general plans, if it may 

 be so expressed, on which all animals seem to have been modelled, 

 and whose ulterior divisions, whatever be the titles with which natu- 

 ralists have decorated them, are merely slight modifications, founded 

 E 



