MAMMALS. 109 



Another series of rodents contains the beaver, common 

 to the Old World and the New, which furnishes furs of 

 great value. These live most of their lives in the water, 

 building dams so that they may always have plenty of it; 

 while their near relatives, the woodchucks, and their west- 

 ern representatives, the prairie-dogs, have no such depend- 

 ence upon water. Highest of all the rodents are the 

 ground - squirrels, the true squirrels, and the flying 

 squirrels. 



ORDER III. INSECTIVORA (Insect-eaters). 



These are small forms, in which all four types of teeth 

 are developed, and which are marked off from all other 

 orders by characters rather difficult of expression. As 

 their name implies, they feed largely upon insects, but 

 worms and other small animals are not despised. The 

 species are largely tropical, but the shrews and moles are 

 found in cooler climates. Most of the species are noctur- 

 nal and burrowing animals, consequently their eyes are 

 small, while their fore legs are adapted for digging. 



ORDER IV. CHEIROPTERA (Bats). 



The bats are the only mammals which truly fly. In the 

 rv.ise of the flying squirrel and the rest, the animals glide 

 Liirougli the air on the plane formed by the lower surface of 

 the body, the tail, and the broad membrane which extends 

 between the limbs; and they can never ascend to the level 

 from which the flight started. With the bats, on the other 

 hand, there are no such limitations to the flight. The wing 

 in the bats consists of a very thin membrane supported 

 upon a framework composed of the body and the bones of 

 the fore limbs. These latter are elongated, four of the 

 fingers excessively so ; and between these fingers and ex- 



