CHAPTER XIV. 



GEOGRAPHICAL VICISSITUDES OF PLANTS. 



Only those familiar with the geological history of 

 Great Britain are aware of the numerous, extreme, 

 and extensive geographical and climatal changes 

 which have taken place in these latitudes since our 

 familiar terrestial plants first appeared. With slow, 

 but certain movements, extending over hundreds of 

 thousands of years, pendulum -like the climate has 

 swung from one extreme to another — from tropical 

 heat to Arctic cold. 



The period in which we live is, as regards our 

 northern hemisphere, half-way between these ex- 

 tremes, or temperate in its climate. We are passing 

 from the last extreme condition of Arctic cold (during 

 the Glacial Period), slowly, to an infinitesimally 

 increasing state of warmth, which may culminate 

 ages hence, in a tropical warmth similar to that 

 which geologists are perfectly assured existed in this 

 country during the Eocene Period of Tertiary time, 

 when the familiar blue clay cut into during the 



