2 
MAMMALS. 
To arrive, therefore, at a correct idea of the mutual relations and affinities of 
animals, and thus to formulate a natural scheme of classification, it is absolutely 
essential to have a certain knowledge of their internal anatomy, as well as of their 
external appearance and their habits. Since, however, such intimate knowledge 
can only be attained after a protracted course of study quite impossible for the 
majority of persons to undertake, it is unavoidable that they must receive a good 
deal on trust from those who have devoted their time to such studies. And yet, 
with a certain amount of attention, every reader should be able to comprehend 
some of the main and leading characters in the structure of animals, by means of 
which they are classified and arranged in a series, which may either commence 
(as in this work) with the highest and descend to the lowest, or may take the 
opposite direction. 
Species and In regard to classification, we commonly divide animals into what 
Genera. the zoologist terms species. Thus all the individuals of the animal 
we call a rat constitute a species, while all those to which we apply the name 
mouse form a second species. The rat and the mouse are, however, obviously 
closely allied species, and are accordingly grouped together by the zoologist as a 
genus, in this particular instance termed Mas. In the large majority of instances 
there is no great difficulty in deciding what is a species, but opinions may 
legitimately differ as to what amount of variation between particular species is 
necessary in order that they should be assigned to different genera. 
Besides the rat and the mouse, there are, however, found in many parts of 
the world certain other animals, known as voles, which differ so markedly from 
rats and mice as clearly to form a distinct genus (in this case termed Microtus), 
although allied to them in so many points as to show that they are very 
near relations. 
Families and Such nearly related groups of species or genera are accordingly 
Orders. grouped together in a family, which takes its name from one of the 
component genera. We, accordingly, have the genera Mas and Microtus con¬ 
stituting the family Muridce; but the number of genera in a family may be 
much larger than this, while in a few instances a family is represented by one 
genus only. 
All, however, who have ever observed with any ordinary attention such 
animals as rats, hares, beavers, and guinea-pigs, will have noticed that there is a 
general similarity in their outward appearance, and that all of them have a single 
pair of chisel-like teeth in both the upper and lower jaw, with which they are in 
the habit of gnawing their food, or any obstacles they may wish to remove. 
Animals thus obviously related to another, although differing in other respects too 
widely to be included in a single family, constitute an order, or assemblage of 
families; the animals to which we have alluded forming the order of Rodents, or 
Rodentia, which will thus include the families Leporidce (hares and rabbits), Muridce 
(rats and mice), CastoridcB (beavers), Caviidce (guinea-pigs), and many others. 
Our grouping by no means ends, however, with an order , for we 
find that groups of orders, from the possession of one or more common 
characters by all of them, may be brigaded together as classes. Thus the Rodents, 
Classes. 
