XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 353 



entered by tlie route of Kamchatka and Alaska, where the 

 climate, even now so much milder and more equable than on 

 the north-east of America, might have been warm enough in 

 late Pliocene times to have allowed the migration of these 

 animals. In Asia they Avere driven southwards by the 

 competition of numerous higher and more powerful forms, 

 but have found a last resting-place in the swampy forests of 

 the Malay region. 



What these Facts Prove. 



Now these two cases, of the marsupials and the tapirs, 

 are in the highest degree instructive, because they show | 

 us that, without any hypothetical bridging of deep oceans, \ 

 and with only such changes of sea and land as are indi- 

 cated by the extent of the comparatively shallow seas 

 surrounding and connecting the existing continents, we are 

 able to account for the anomaly of allied forms occiu-ring i 

 only in remote and widely separated areas. These examples 

 really constitute crucial tests, because, of all classes of animals, 

 mammalia are least able to surmount physical barriers. They 

 are obviously unable to pass over wide arms of the sea, 

 while the necessity for constant supplies of food and water 

 renders sandy deserts or snow -clad plains equally impass- 

 able. Then, again, the peculiar kinds of food on which 

 alone many of them can subsist, and their liability to the 

 attacks of other animals, put a further check upon their 

 migrations. In these respects almost all other organisms 

 have great advantages over mammals. Birds can often fly 

 long distances, and can thus cross arms of the sea, deserts, or 

 mountain ranges; insects not only fly, but are frequently 

 carried great distances by gales of wind, as shown by the 

 numerous cases of their visits to ships hundreds of miles from 

 land. Keptiles, though slow of movement, have advantages in 

 their greater capacity for enduring hunger or thirst, their power 

 of resisting cold or drought in a state of torpidity, and they 

 have also some facilities for migration across the sea by means 

 of their eggs, which may be conveyed in crevices of timber or 

 among masses of floating vegetable matter. And when we 

 come to the vegetable kingdom, the means of transport are 

 at their maximum, numbers of seeds having special adaptations 



2 a 



