176 



THE GNAWERS OR RODENTS. 



of Asia, the other four being restricted to 

 Australia. Since the latter island is other- 

 wise inhabited only by non-placental mam- 

 mals, the presence of a few rodents cannot 

 but be regarded as a phenomenon all the 

 more striking. The island of Madagascar, 

 on the other hand, occupies an altogether 

 exceptional position from another point of 

 view, inasmuch as it is peculiarly rich in 

 other placental mammals, and contrasts with 



other regions also in the peculiarity of its 

 rodents. 



The other regions of distribution exhibit 

 very unusual relations. South America is 

 the richest of all the continents in rodents 

 and that which has the most specialized 

 forms. Of the thirty-seven genera repre- 

 sented there, thirty-two belong to it exclu- 

 sively, three it shares with North America, 

 and the other two genera, those of the hares 



.\]l)ine or Mountain Hare [Lcptis alpinus). page 174. 



and rabbits and the squirrels, are genera 

 which are found all the world over. It is 

 manifest, accordingly, that South America 

 formed an independent centre of evolution, 

 whose primitive stocks were dififerent from 

 those of the other parts of the earth. 



Such also is the case with the true continent 

 of Africa, which, considered from a zoological, 

 botanical, and geological point of view, com- 

 prises only the region to the south of the 

 Sahara. Yet, though this region is almost 

 as rich in forms as South America (there are 

 in it thirty-one genera), it is far from present- 

 ing the same degree of specialization; for it 

 has four genera besides the hares and squirrels 

 in common with the Mediterranean region. 



other four in common with the East Indies, 

 so that only twenty -one belong to it ex- 

 clusively. 



The Palaearctic Region, notwithstanding 

 its enormous extent, including as it does all 

 the area to the north of the Atlas Mountains 

 and the Himalayas, is more sparingly supplied 

 with rodents than either of the two previously 

 mentioned regions. India being left out of 

 account, Europe and Asia together have only 

 nine, out of twenty-five genera represented 

 in them, peculiar; the other sixteen are 

 shared either with Africa, India, or North 

 America. As in so many other cases, we 

 can in the case of the rodents also verify the 

 fact that the polar regions have a large 



