. Introduction to zoology; a guide to the study of animals, for the use of secondary schools;. continu-ously throughout life, and are not formed, once for all, likeour teeth. In addition to plant materials, rats and miceeat a certain amount of animal food. The different races of tame mice illustrate the resultsobtained by preserving sports and selecting the best forbreeding purposes. The white mouse, for instance, is an 320 ZOOLOUY albino ; that is, an animal without pigment. Albinos oc-cur among almost all kinds of mammals, but the cause ofthem is unknown. So, likewise, the black race of tame
. Introduction to zoology; a guide to the study of animals, for the use of secondary schools;. continu-ously throughout life, and are not formed, once for all, likeour teeth. In addition to plant materials, rats and miceeat a certain amount of animal food. The different races of tame mice illustrate the resultsobtained by preserving sports and selecting the best forbreeding purposes. The white mouse, for instance, is an 320 ZOOLOUY albino ; that is, an animal without pigment. Albinos oc-cur among almost all kinds of mammals, but the cause ofthem is unknown. So, likewise, the black race of tamemice is due to an excess of pigment, a sport found nowand then among other mammals. The pure reddish brownraces have probably been produced by selecting the housemice with little black in the fur. By preserving occasionalsports and by selecting for breeding the purest-coloredindividuals, all the different races of our domestic animalsand 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. Fi<?</, the Georgia gopher. One-half nat. size. AfterV. Bailey. the cutting teeth grow continuously throughout life, a con-dition associated with the habit of gnawing. This groupis very abundantly represented in North America, moreso, indeed, than in other continents. All our rodents fallinto seven families: (1) the squirrels, gophers (Fig. 295),prairie dogs, and wood chuck, with long and hairy tail ;(2 ) the beaver, with broad and scaly tail ; (3) the pouchedgophers, with peculiar cheek pouches opening outside the 1 From roder f, to gnaw. THE MOUSE AND ITS ALLIES 321 mouth; (4) the jumping mice, with greatly elongatedhind legs; (5) the muskrat and the various wood andfield mice, closely related to the genus Mus ; (6) the por-cupines, with bristles replacing hairs; (7) the hares, withlong ears and short tails. All of these familiar rodentsare, on the whole, destructive to agriculture.
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