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 IMamraals — 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 arc 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 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 WTiters, 

 and it wnll 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 



