CHAPTER Y. 
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 Mollusca — 
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 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 
