310 Cuvieria7i System of Zoology. 



The descriptions of the different classes and orders of ani- 

 mals in the following essays will be illustrated by references 

 to cuts, and such explanations will be added as, I trust, will 

 be generally intelligible. 



The definition which Cuvier gives of the living principle is 

 rather a description of its effects than of its essence, which 

 must ever remain an impenetrable mystery. It^ says he, 

 we would form a just idea of the essence of life, and observe 

 it in those beings where its effects are the most simple, we 

 shall perceive that it consists in the faculty which certain or- 

 ganised bodies have of existing for a certain time, under a 

 determinate form, by incessantly absorbing into their own 

 composition a part of the surrounding substances, and by 

 giving back to the elements a portion of their own proper 

 substance. Life is, therefore, a more or less complicated agi- 

 tation or vortex {tourhillon\ constantly going on in a certain 

 direction, and carrying with it molecules of the same kind, but 

 into which other individual molecules are continually entering 

 or passing out; so that the^rw of the living body is more 

 essential to it than the matter. When this vital motion finally 

 ceases, the body dies, and the elements of which it was com- 

 posed separate and are again subjected to common chemical 

 affinities, and decomposition speedily takes place. Hence, it 

 appears, that the living principle possesses the important power 

 of suspending or changing the laws of chemical affinity; for 

 it was this power that prevented the putrefaction or decom- 

 position of the living body, and united its elementary mole- 

 cules. 



All living bodies die after a period, the extreme term of 

 which is fixed for each species. Death appears to be a neces- 

 sary effect of life, for the vital motion insensibly changes the 

 bodies in which it acts, so as to render their permanent con- 

 tinuance impossible. 



The living body experiences gradual but continual changes 

 during its whole existence. At first it increases in size, accord- 

 ing to certain proportions and limits fixed for each species, 

 and for each of its parts. After the parts are full grown, they 

 increase in density. It is this second change which appears 

 to be the cause of natural death. 



The process which terminates in what Cuvier describes as 

 natural death, consists not only in the increase of density, but 

 of hardness, and goes on until the fibres and vessels are con- 

 verted into bone : it is called ossification, and has been well 

 described by Dr. Armstrong : — 



" Life glows meantime amid the grinding force 

 OF viscous fluids and elastic tubes ; 



