MAMMALS 7 



And, just as a general can tell to what company, regiment, 

 brigade, and division a soldier belongs, by looking at his uniform, 

 so a naturalist can tell to what genus, family, order, class, and 

 division an animal belongs, by looking at its bodily characteristics. 



THE VERTEBRATES 



MAMMALS 



The animal army, as we have seen, is divided by naturalists 

 into two great halves — the Vertebrates and Invertebrates. The 

 principal difference between these two divisions is, that every 

 Vertebrate possesses bones and a spinal column of some kind 

 or other, while the Invertebrates have no inside skeleton at all. 

 In the Invertebrates the hard parts are, when they exist, usually 

 placed outside the body; as, for instance the shell of the crab or 

 lobster or snail, or the horny covering of beetles or other insects. 

 Another difference is, that the Vertebrates have never more than 

 four limbs, while the Invertebrates may possess a large number — 

 in some cases amounting to hundreds. Lastly, every vertebrate 

 animal possesses a distinct heart, which in many of the Inverte- 

 brates is absent. There are other differences between these two 

 great divisions with which we need not trouble ourselves at present. 



Mammals are the highest of the five classes that form the 

 Vertebrate division. By highest we mean that the bodies of the 

 Mammals contain as a rule a larger number of parts, each 

 intended for a special use, than those of the other four classes — 

 birds, reptiles, batrachians, and fishes. 



The Mammals vary greatly in size and shape, from the tiny 

 mouse to the huge whale, which, as we shall see later on, is not 

 a fish, but a Mammal living in the water. But whatever their 

 differences, all Mammals have certain points of resemblance to 

 one another. In the first place, they suckle their young; secondly, 



