GEOGRAPHICAL VICISSITUDES OF PLANTS. 289 



the islands would be better circumstanced, and also 

 free from invasion. Now let ages elapse, and an 

 upward movement once more connect the island with 

 the mainland, so that it formed an integral part 

 of it — we should then have the phenomenon of two 

 distinct floras living side by side. 



Or let the opposite fact be imagined. Islands 

 are formed during a Glacial Period, and the char- 

 acteristic flora remains with them. The climate 

 alters ; some of the more sensitive species die out 

 (although plants used to cold adapt themselves 

 sooner to increased warmth than those which love 

 heat do to cold) ; others creep farther up the hill- 

 sides to the summits. When the intervening sea- 

 bed is upheaved, these hilltops will have been raised 

 still higher above the sea-level and into a colder 

 region of the atmosphere. Or suppose a country 

 to be very cold and occupied by an Arctic flora, 

 and its geographical connections such that it is 

 part and parcel of an extensive continent, or series 

 of continents (like the land-masses of the northern 

 hemisphere). Then, if the climate slowly changes 

 to warmer conditions, as the snows cease to lie per- 

 manently on the mountain-slopes, and to fill up the 

 valleys with their glaciers, the cold -loving plants 

 of the plains, finding the climate changing, would 

 creep up to higher and therefore colder levels, and 

 occupy the ground previously covered with snow and 

 ice. Eventually the tops of all the high hills and 



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