INTRODUCTION. y 
class contains orders, and the orders are sub- 
divided into families; each family is subdivided 
into genera, and each genus contains one or 
more species. The animal kingdom is composed 
of individuals; but among these there are a certain 
number which have a close resemblance to each 
other, and are recognizable by a character which 
is constant and definite. Such groups of indi- 
viduals constitute what naturalists call species. 
In familiar language, we speak of these in such 
general terms as the horse, the oyster, the garden 
snail, &c.,—meaning no horse in particular, no 
oyster, no garden snail in particular. Let us 
examine this matter more closely. Let us collect 
a number of snails from a garden; we see that 
they form a natural group, for they are all 
characterized by a yellowish-coloured shell, beau- 
tifully banded with brown. This assemblage of 
individuals with like characters we call ‘the 
garden snail.” Let us generalize still further 
and higher, and extend our sphere of collection 
to the neighbouring wood. We now find that 
there are other undoubted snails, but which 
evidently form a group distinct from “ the 
garden snail,’”’ for all their shells are of much 
smaller size, and are clothed with hairs ; from 
which latter character we may appropriately call 
them “the hairy snail.”” Observe that we have 
naturally spoken of these two groups under the 
