INTRODUCTION. y | 
abundance or scarcity of food, differences of 
habitat, &c. Thus, the shell of the large pond 
snail, Limneea 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 sus- 
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. 
