320 ZOOLO<; r 



albino ; that is, an animal without pigment. Albinos oc- 

 cur among almost all kinds of mammals, but the cause of 

 them is unknown. So, likewise, the black race of tame 

 mice is due to an excess of pigment, a sport found now 

 and then among other mammals. The pure reddish brown 

 races have probably been produced by selecting the house 

 mice with little black in the fur. By preserving occasional 

 sports and by selecting for breeding the purest-colored 

 individuals, all the different races of our domestic animals 

 and plants have been established and improved. 

 . Other Rodents. The mouse belongs to the group Ro- 

 dentia, 1 distinguished from other mammals by the fact that 



FIG. 295. Geomys tuza, the Georgia gopher. One-half nat. size. After 

 V. Bailey. 



the cutting teeth grow continuously throughout life, a con- 

 dition associated with the habit of gnawing. This group 

 is very abundantly represented in North America, more 

 so, indeed, than in other continents. All our rodents fall 

 into seven families: (1) the squirrels, gophers (Fig. 295), 

 prairie dogs, and woodchuck, with long and hairy tail; 

 (2) the beaver, with broad and scaly tail ; (3) the pouched 

 gophers, with peculiar cheek pouches opening outside the 



1 From rodere, to gnaw. 



