IN NATURAL HISTORY 5 



by their nature, but only by their range ; they 

 were still to him, as genera and species had been 

 to Aristotle, only larger or smaller groups, not 

 founded upon and limited by different categories 

 of structure. He divided the animal kingdom 

 into six classes, which I give here, as we shall 

 have occasion to compare them with other clas- 

 sifications : Mammalia, Birds, Reptiles, Fishes, 

 Insects, and Worms. 



That this classification should have expressed 

 all that was known, in the last century, of the 

 most general relations among animals, only shows 

 how difficult it is to generalize on such a sub- 

 ject ; nor should we expect to find it an easy task, 

 when we remember the vast number of species 

 (about a quarter of a million) already noticed 

 by naturalists. Linnaeus succeeded, however, in 

 finding a common character on which to unite 

 most of his classes ; but his definition of the 

 class of Mammalia, that group to which we our- 

 selves belong, remained very imperfect. Indeed, 

 in the earlier editions of his classification, he 

 does not apply the name of Mammalia to this 

 class, but calls the higher animals Quadrupedia, 

 characterizing them as the animals with four legs 

 and covered with fur or hair, that bring forth 

 living young and nurse them with milk. In 

 thus admitting external features as class char- 

 acters, he excluded many animals which by their 



