CHAP. V DISPERSAL OF ANIMALS AND PLANTS 73 



particular cases that arise in regard to the faunas and floras 

 of remote islands. But the other side of the question of 

 dispersal — that which depends on geological and cUmatal 

 changes — is in a far less satisfactory condition, for, though 

 much has been written upon it, the most contradictory 

 opinions still prevail, and at almost every step we find 

 ourselves on the battle-field of opposing schools in 

 geological or physical science. As, however, these 

 questions lie at the very root of any general solution of 

 the problems of distribution, I have given much time to a 

 careful examination of the various theories that have been 

 advanced, and the discussions to which they have given 

 rise ; and have arrived at some definite conclusions which 

 I venture to hope may serve as the foundation for a better 

 comprehension of these intricate problems. The four 

 chapters which follow this are devoted to a full examin- 

 ation of these profoundly interesting and important 

 questions, after wdiich we shall enter upon our special 

 inquiry — the nature and origin of insular faunas and 

 floras. 



The Ocean as a Barrier to the Dispersal of Mammals. — A 

 wide extent of ocean forms an almost absolute barrier to 

 the dispersal of all land animals, and of most of those 

 which are aerial, since even birds cannot fly for thousands 

 of miles without rest and without food, unless they are 

 aquatic birds which can find both rest and food on the 

 surface of the ocean. We may be sure, therefore, that 

 without artificial help neither mammalia nor land birds can 

 pass over very wide oceans. The exact width they can 

 pass over is not determined, but we have a few facts to 

 guide us. Contrary to the common notion, pigs can swim 

 very well, and have been known to swim over five or six 

 miles of sea, and the wide distribution of pigs in the islands 

 of the Eastern Hemisphere may be due to this power. It 

 is almost certain, however, that they would never 

 voluntarily swim away from their native land, and if 

 carried out to sea by a flood they would certainly endeav- 

 our to return to the shore. We cannot therefore believe 

 that they would ever swim over fifty or a hundred miles of 

 sea, and the same may be said of all tlie larger mammalir.. 



