THE VERTEBRATE ANIMALS 



317 



man, by artificial selection, might have considerably modified the 

 type of horse with which he started. In this manner have been 

 established and improved the various types of horses familiar 

 to us as draft horses, coaches and hackneys, and the trotters. In 

 a similar manner have been obtained the various breeds of cattle, 

 sheep, swine, etc. 



It is needless to say that all the various domesticated animals 

 have been tremendously changed in a similar manner since civilized 

 man has come to live on the earth. When we realize the very 

 great amount of money invested in domesticated animals; that 

 there are over 60,000,000 each of sheep, cattle, and swine and 

 over 20,000,000 horses owned in this country, then we may see 

 how very important a part the domestic animals play in our lives. 



Other Orders of Mammals. The lowest are the monotreraes, animals 

 which lay eggs like the birds, although they are provided with hairy 

 covering like other mammals. 

 Such arc the Australian spiny 

 ant rater and the duck mole. 



All other mammals bring forth 

 their young developed to a form 

 similar to their own. The kan- 

 garoos and opossum, however, 

 are provMod with a pouch on 

 the ventral side of the body in 

 which the very immature, blind, 

 and helpless young are nour- 

 ished until they are able to care Virginia opossum. Photograph, one eighth 

 for themselves. These pouched natural size, by N. F. Davis, 



animals are called marsupials. 



The other mammals, in which the young are born able to care for 

 themselves, and have the form of the adult, may be briefly classified as 

 follows : 



CHARACTER 



Edentates Toothless or with very simple 

 teeth 



Rodents Incisor teeth, chisel-shaped, usu- 

 ally two above and two 



below 



Cetaceans Adapted to marine life, teeth of 

 whales sometimes platelike 



EXAMPLES 



Anteater 

 Sloth 

 Armadillo 

 Beaver, rat 

 Porcupine, rabbit 

 Squirrels 

 Whales 

 Porpoise 



