GEOGRAPHICAL VICISSITUDES OF PLANTS. 287 



conditions — a change commenced, which ultimately 

 required the whole process to be done over again. 

 A warmth-loving flora, like that we know existed in 

 England during the Eocene Period, had settled down 

 pleasantly, and seemed to be irremovable. This 

 state of things must have existed for many hundreds 

 of thousands of years, when the climate gradually 

 toned down, more and more, during the Miocene 

 Period ; and, when the succeeding Pliocene era set in, 

 it inaugurated a temperate climate like that now 

 prevailing. This was followed by the long, bitter 

 climatal winter of " The Great Ice Age." Even it 

 the warmth-loving Eocene plants had been able to 

 adapt themselves to the lower temperature, they could 

 not have lived in Great Britain during the latter 

 period, for during one stage, at least, of that bitter 

 time, these islands were swathed in an ice-sheet just 

 as Greenland is now, and this would effectually pre- 

 vent plants from maintaining their ancient habitats. 



The case must have gone equally bad with 

 them, even if the climate remained practically un- 

 changed, when one of the numerous oscillations of 

 level occurred which lowered the dry land until it 

 became the bed of a sea. 



In both these instances there would be only one 

 way in which plants could perpetuate their species 

 when their ancient sites had been taken from them 

 — they would have to viigrate. The British plants 

 of the Eocene Period, under stress of altered climate, 



