2 CLASSIFICATION. 



A disbanded army presents to the observer nothing 

 but a wild scene of inextricable confusion ; but when 

 at tnimpet-call, the soldiers hasten to their ranks, 

 and the appropriate banner waves above each com- 

 pany, these companies fall into regiments, and the 

 living mass, directed by one chief, moves on with 

 the utmost order and regularity. 



Systematic arrangement is, therefore, the very 

 foundation of the science of zoology : it is only by 

 the establishment of classes, and orders, and genera, 

 and species, which constitute, so to speak, the colours 

 of the different regiments, that such arrangement is, 

 at all, to be accomplished, and to define the limits 

 and the characters of these genera and species, the 

 efforts of the scientific zoologist are principally di- 

 rected. It must, consequently, be our first endeavour 

 to explain what these words, species and genera, mean. 



By Species is understood a number of animals so 

 closely resembling each other, that they all might 

 be supposed to be the offspring of the same parents, 

 and in turn to give birth to progeny, exactly resem- 

 bling themselves. The domestic mouse, for example, 

 is a species, the exact fac-simile both of its ancestors 

 and its offspring. Species, however, may be slightly 

 modified by the continued operation of external cir- 

 cumstances, such as climate, abundance or deficiency 

 of food, or other similar accidents ; there may, for 

 example, be a white mouse, or a piebald mouse ; 

 these are called Varieties of the species. 



A Genus is a group embracing a number of species 

 which have a striking general resemblance to each 

 other in certain important particulars, whereby they 

 are distinguishable from all other animals. The 

 domestic mouse {Mus musculus), for instance, is at 

 once recognisable from the squirrel, the beaver, or the 

 hare, from the circumstance that it has a long tail 

 tapering to a point and denuded of hair ; but there 

 are many other animals which, though evidently not 

 real mice, have this feature in common. There is 

 the rat, Mus rattv.s ; the brown rat, Mus decumanus ; 



