INTRODUCTION. 
7 
abundance or scarcity of food, differences of 
habitat, &c. Thus, the shell of the large pond 
snail, Limncea stagnalis , becomes more length¬ 
ened, tapering, and thinner, when the animal 
lives in running water, with only vegetable food 
as a diet, than the shells of more favoured indi¬ 
viduals inhabiting stagnant ponds, which fare 
more sumptuously upon dead dogs and other 
animals. 
Classification is, then, an arrangement of all 
beings according to a certain order, by means 
of which objects are reunited into groups, recog¬ 
nizable by determinate characters, which, in 
their turn, are reunited into other groups of a 
still more comprehensive character. We have 
thus the animal kingdom subdivided into sub¬ 
kingdoms ; these sub-kingdoms are further sub¬ 
divided into classes; and these classes, again, 
into ORDERS, FAMILIES, GENERA, and SPECIES. 
“The practical utility of such a classification is 
easily seen by comparing it with the address of 
a letter. So it is with the naturalist, who by 
his classifications arrives speedily to the groups 
to which the animal belongs. 
If, for example, he wished to define a garden 
snail , without resorting to such means, he would 
be forced to compare his description with that of 
* M. Milne-Edwards, 
