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. 4 
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. Linneus 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 
