THE FUNCTIONS OF LIFE. 49 



which lead us to class them into more or less 

 comprehensive groups. Thus in the animal 

 kingdom, quadrupeds, birds, fishes, reptiles, 

 shell-fish, and insects, compose natural assem- 

 blages or classes, and each of these is readily 

 divisible into subordinate groups or families. 

 Now it results from a closer examination of the 

 structure and economy of plants and animals, 

 that the formation of all the individual species 

 comprehended in the same class, has been con- 

 ducted in conformity with a certain ideal model, 

 or type, as it is called. Of this general type all the 

 existing forms appear as so many separate copies, 

 differing, indeed, as to particulars, but agreeing 

 as to general characters. The same observation 

 applies to the families, the genera, and other 

 subordinate groups of living beings. 



The more extensive our acquaintance is with 

 the anatomy and physiology of both plants and 

 animals, the more striking do these analogies 

 appear ; so that amidst endless diversity in the 

 details of structures and of processes, the same 

 general purpose is usually accomplished by simi- 

 lar organs and in similar modes. So firmly is 

 this principle established, that we may venture 

 with confidence to predict many circumstances 

 relating to an unknown animal, of which only 

 a few fragments are presented to us, from our 

 general knowledge of the characters and economy 

 of the tribe or family, on the type of which it has 



VOL. I. E 



