INTRODUCTION. 
0 
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 
