256 FIRST LESSONS IN ZOOLOGY 



tail. Their strong hind legs enable them to stand erect, 

 and even to take several steps in this posture. They 

 can swim readil}', although naturally the}- rarely take to 

 water. Their special senses are keen, the senses of hear- 

 ing and taste being unusually well developed. Their 

 ' ' singing, ' ' which has been the subject of much discussion, 

 seems to be actually a voluntary and normal performance, 

 which, however, hardly deserves to be called singing, but 

 rather a slightly varied peeping or whistling. 



The mouse is a prolific mammal, producing from four 

 to six times a year broods of from four to eight young. 

 A cozy nest of straw, bits of paper, feathers, wool, or 

 other soft materials is made, and in this the j'oung are 

 born. The newly born mice are very small and are 

 blind and helpless. They are odd little creatures, being 

 naked and almost transparent. They grow rapidl)', 

 being covered with hair in a week, although not opening 

 their eyes for about two weeks. A day or two after 

 their eyes are open they begin to leave the nest, and 

 hunt for food for themselves. 



Classification. — The mammals of North America rep- 

 resent eight orders. Three additional orders, namely, the 

 Monotremata, including the e.xtraordinary duckbills of 

 Australia, the Edentata, including the sloths, armadillos 

 and ant-eaters found in tropical regions, and the Sirenia, 

 including the marine manatees and dugongs are not rep- 

 resented (except by a single manatee) in North America. 

 In the following paragraphs some of the more familiar 

 mammals representing each of the eight orders repre- 

 sented in North America arc referred to. 



The opossums and kangaroos (Marsupialia). — The 

 opossum (Didclpliys virginiaiia) is the onl_\' North Amer- 

 ican representative of the order Marsupialia, the other 

 members of which are limited exclusively to ^Australia 

 and certain neighboring islands. The kangaroos are the 



