398 



ZOOLOGY 



Placental 

 mammals 



Photograph by E. R. Sanborn, 

 N. Y. Zool. Soc. 



FIG. 170. Great gray kangaroo (Macro- 

 pus giganteus). 



There are other charac- 

 teristic features of the 

 skeleton and teeth, but 

 within the limits of the 

 Marsupialia we find the 

 greatest diversity of out- 

 ward form and of habits. 

 Nature, as in so many 

 cases, produces species 

 adapted to all sorts of 

 life and consequently su- 

 perficially resembling 

 others which have paral- 

 lel habits but are not at 

 all closely related. Australia is the present home of 

 marsupials, but America also possesses examples, the 

 most familiar being the opossum. The opossums, of 

 which there are several kinds, inhabit both North and 

 South America, living in trees. The survival of so 

 many marsupials in Australia has been possible be- 

 cause the region has been cut off from the rest of the 

 world for ages, and the higher mammals have for 

 the most part failed to reach it. In Australia we find 

 the kangaroos, wombats, phalangers, pouched mole, 

 and many other forms. It used to be said that the 

 Australian marsupials simulated almost every type of 

 land mammal except the mole, and it was a matter 

 of great interest to zoologists when at length a molelike 

 species (Notoryctes) was discovered. 



4. The Placental or higher mammals, including all 

 the most familiar forms, are nourished within the body 

 of the mother, and are born in an advanced state of 

 development. There are numerous orders, of which 

 the following are the most important : 



