CHAP, v.] DISPERSAL OF ANIMALS AND PLANTS. 



71 



question of dispersal — that which depends on geological and 

 climatal 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 com- 

 prehension of these intricate problems. The four chapters 

 which follow this are devoted to a full examination of these 

 profoundly interesting and important questions, after which we 

 shall enter upon our special inquiry — the nature and origin of 

 insular faunas and floras. 



The Ocean as a Bamer 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 

 who 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 endeavour 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 the 

 larger mammalia. Deer also swim well, but there is no reason 

 to believe that they would venture out of sight of land. With 

 the smaller, and especially with the arboreal, mammalia, there is 

 a much more effectual way of passing over the sea, by means of 



