SUMMARY 
139 
Beavers live in colonies and work industriously in building 
dams and houses. Some damage is done by their cutting 
trees and damming up streams. The great popular interest 
in beavers and their works more than outweighs the little 
harm they do. 
Figure 131. — Runways of Muskrat. 
As the water in the pond recedes, the muskrats dig canals that enable 
them to reach water from their lodges in the bank without going over land. 
How does this show adaptation in the behavior of these animals ? These 
canals are often mistaken for beaver runways. 
SUMMARY 
The animals which are called mammals are covered with 
hair and nourish their young with milk. There are nearly 
always two pairs of appendages that undergo much modi¬ 
fication, according to the habits of the animals. Our do¬ 
mestic animals which serve us in so many ways have grad¬ 
ually developed into their present form and usefulness. 
Man had to learn first how to use the fur and skin of wild 
animals, then how to improve the quality of the fur and skin 
by careful feeding and breeding of the domesticated animals. 
