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 direc- 

 tion. 



In regard to classification, we commonly divide animals into what the 

 r 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 Mus. In the large majority of instances 

 there is no great difficulty in deciding what is a species, but opinions may legiti- 

 mately 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 the case termed Microtus}, although 

 allied to them in so many points as to show that they are very near relations. 



Such nearly related groups of species or genera are accordingly 

 Families and , , . ......... . 



Orders grouped together in a family, which takes its name from one of the 



component genera. We, accordingly, have the genera Mus and Mi- 

 crotus constituting 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 ani- 

 mals to which we have alluded forming the order of Rodents, or Rodentia, which 

 will thus include the families Leporida (hares and rabbits), Muridce (rats and mice), 

 Castoridfe (beavers), Caviida (guinea-pigs), and many others. 



Classes ^ ur rou pi n g 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, 



the Hoofed Animals or Ungulates (pigs, deer, cattle, horses, etc.), and the Carni- 



