14 FORMATION OF THE EARTH 



islands, on the other hand, have become united to one another, 

 and then attached to the continents nearest to them ; and the 

 vast regions thus formed have again been divided up by the 

 invading waters. Plants and animal organisms which once 

 lived together have become isolated from one another as a 

 result ; species enclosed in regions separated by seas have been 

 able to spread from one to the other as soon as a land-bridge 

 appeared, and to pass from one sea to another as soon as they 

 were connected by straits. The evolution of life is intimately 

 bound up with these slow and peaceful terrestrial " revolu- 

 tions ", which, in fact, have been merely an evolution which 

 we shall have to study before we can examine the evolution 

 of life. 



