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 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 
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 
