THE FUNCTIONS OF LIFE. 41 



The most superficial survey of nature is suffi- 

 cient to show that there prevail certain general 

 resemblances among great multitudes of species, 

 which lead us to class them into more or less com- 

 prehensive groups. Thus in the animal kingdom, 

 Quadrupeds, Birds, Fishes, Reptiles, Shell-fish, 

 and Insects, compose natural assemblages or classes, 

 and each of these is readily divisible into subordi- 

 nate 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 conducted 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 sepa- 

 rate copies, differing, indeed, as to particulars, but 

 agreeing as to general characters. The same ob- 

 servation 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 pur- 

 pose is usually accomplished by similar 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 pre- 

 sented to us, from our general knowledge of the 

 characters and economy of the tribe or family, on 

 the type of which it has been modelled. Thus the 

 discovery of a mutilated portion of the skeleton of a 

 fossil animal, conveys to the physiologist, who is 



