78 VIGNETTES FROM NATURE, 



upon earth. In the great continents they 

 are almost all dominant mammals ; for 

 example, the elephant, rhinoceros, hippo- 

 potamus, giraffe, and bison. Wherever we 

 get large species of lower mammals or of the 

 inferior classes, they are invariably found 

 either on insulated lands or on lands but 

 recently united to the continents. Thus, the 

 great fossil sloths, armadillos, and other 

 edentates belong to the insular fauna of 

 South America, where no higher mammals 

 existed ; as soon as the mammalian types of 

 the northern continent began to make their 

 way across the quite modern Isthmus of 

 Panama, all the bigger native forms became 

 extinct. Just in the same way, Australia still 

 possesses a very large marsupial in the great 

 kangaroo ; but if the Australian region had 

 ever been joined to Asia, the Asiatic car- 

 nivores would soon have exterminated this 

 stupid and defenceless herbivore. So, too, the 

 moa was developed in New Zealand, where 

 there were no mammals at all, and where the 



