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Show CHAPTER V. THE POWERS OF DISPERSAL OF ANIMALS AND PLANTS. Statement of the general question of Dispersal-The Ocean as a barrier to the dispersal of Mammals-The dispersal of Birds-The dispersal of Reptiles-The dispersal of Insects-The dispersal of Land MolluscaGreat antiquity of Land-shells-Causes favouring the abundance of Land-shells-The dispersal of Plants-Special adaptability of Seeds for dispersal-Birds as agents in the dispersal of Seeds-Ocean currents as agents in Plant dispersal-Dispersal along mountain-chains-Antiquity of Plants as affecting their distribution. IN order to understand the many curious anomalies we meet with in studying the distribution of animals and plants, and to be able to explain how it is that some species and genera have been able to spread widely over the globe, while others are confined to one hemisphere, to one continent, or even to a single mountain or a single island, we must make some inquiry into the different powers of dispersal of animals and plants, into the nature of the barriers that limit their migra.tions, and into the character of the geological or climatal changes which have favoured or checked such migrations. The first portion of the subject-that which relates to the various modes by which organisms can pass over wide areas of sea and land-has been fully treated by Sir Charles Lyell, by Mr. Darwin, and many other writers, and it will only be necessary here to give a very brief notice of the best known facts on the subject, which will be further referred to when we come to discuss the particular cases that arise in regard to the faunas and floras of remote islands. But the other side of the CUAP. V.] DISPERSAL 0 F 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 comprehension 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 shaH enter upon our special inquiry-the nature and origin of insular faunas and floras. :the Ocean as a Bctrrier to the Dispe?'Sal 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 S\\·im 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 sma1ler, and especially with the arboreal, mammalia, there is a much more effectual way of passing over the sea, by means of |