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 

 Keptiles — The dispersal of Insects — The dispersal of Land Molliisca — 

 Great 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 con- 

 fined 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 migrations, and into the 

 character of the geological or climatal changes v/hich 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 



