INTRODUCTION 93 



Laurels, etc., grew in this country where we have 

 to-day a temperate flora. 



The origin of the British flora is as diflicult to solve 

 as that of the origin of Angiosperms. The change 

 of climate after the Eocene brings us to Pliocene 

 times when it was much colder, and in the pre-Glacial 

 flora we have many plants that are living to-day along 

 with some southern types such as the Water Chestnut 

 of the Mediterranean, which are no longer found here. 



With the Glacial epoch arctic conditions were 

 introduced. There had been anterior to this in the 

 arctic regions a flora of a tropical character in 

 Greenland and elsewhere, and this flora was swept 

 away before Pliocene times. But it may have spread 

 to Europe from America, where in Cretaceous times a 

 rich Angiospermic flora flourished, to be followed 

 by a later, equally rich Tertiary flora via the northern 

 regions if we cannot postulate then a land bridge or 

 " Atlantis " between Europe and America. 



When, at any rate, colder conditions prevailed. 

 Pliocene, and later Pleistocene modern types were 

 evolved in America and in Europe.* In this area 

 the southern plants which spread from the Mediter- 

 ranean area northwards were driven southwards by 

 the Ice Age, whilst northern arctic plants came south 

 with arctic animals such as the Mammoth and the 

 Reindeer. During this Glacial period there were 

 milder Interglacial periods. Some of the plants of 



* We know less generally of the floras of the Southern Hemisphere 

 than of the Northern. 



