II MAMMALS OF ARCTIC REGION 8 1 



that but little reliance can be placed on them as a means of 

 transit ; besides, here again, two individuals, or a pregnant female, 

 would be required to effect a settlement on a foreign shore. 

 The existence of oceanic islands is often urged as a proof of this 

 inability to cross tracts of sea ; even those which are com- 

 paratively near an extensive continent, such as, for example, 

 Fernando iSToronha in the Atlantic, are destitute of mammals 

 (except, indeed, the ubiquitous ilouse, which is believed to have 

 been carried there, often in company with the equally widely- 

 spread Hat, in ships). This argument, however, is not so conclusive 

 as might appear ; it doubtless is in the case of far-distant islands. 

 But the size of the islands has to be taken into account. For 

 there are islands, such as the G-alapagos, or, to take a less con- 

 tested instance, some of the islands of the Malagasy Archipelago, 

 undoubtedly continental, which have an exceedingly reduced 

 number of mammals. An area of a certain size seems to be a 

 necessity. 



The converse of this is in many cases easy to shoWj that is, 

 the wide range of animals when there are no marine barriers to 

 stop their spreading. John Hunter, the celebrated anatomist 

 and surgeon (not often quoted, however, as an authority upon 

 geographical distribution), observes : " It is a curious circum- 

 stance in the natural history of animals to find most of the 

 northern animals the same both on the continent of America and 

 what is called the Old World, while those of the warmer parts 

 of both continents arp not so. Thus we find the bear, fox, wolf, 

 elk, reindeer, ptarmigan, etc., in the northern parts of both. . . . 

 The reason why the same animals are to be found in the northern 

 parts is the nearness of the two continents. They are so near 

 as to be within the power of accident to bring the animals, 

 especially the large ones, from one continent to the other either 

 on the ice or even by water. But the continents diverging from 

 each other southward, so as to be at a very considerable distance 

 from each other even beyond the flight of birds, is the reason 

 why the quadrupeds are not the same." 



There is no doubt, in fact, that the ocean is the most in- 

 superable of all barriers to the dispersal of mammals. In a less 

 degree mountain ranges and deserts are also barriers. The 

 Desert of Sahara is a striking instance to the point ; it separates 

 two exceedingly different faunas. 



VOL. X G 



