THE SIBERIAN MIGRATION. 239 



Betnla nana and B. alba, Salix cinerea and Arctosta- 

 phylos uva-ursi. 



Now three of these four species of plants are still 

 natives in the British Islands, and all are forms 

 which probably came to us with the Arctic migration 

 which I described in Chapter IV. They travelled 

 south with the reindeer, or before it, and may have 

 covered large tracts of country at the time. With the 

 increased struggle for existence on the arrival of the 

 Siberian and Oriental migrants, they have probably 

 been evicted by these more powerful rivals. A 

 discovery of their remains does not necessarily 

 indicate that a great change of climate has taken 

 place since they lived in the country. And certainly 

 these Arctic plants cannot be taken as indicating a 

 low temperature, for it has been shown that Alpine 

 plants are mostly intolerant of very low temperatures. 

 " Arctic and Alpine species in the Botanical Gardens 

 at Christiania," says Professor Blytt (p. 19), "endure 

 the strongest summer heat without injury, while they 

 arc often destroyed when not sufficiently covered 

 during the winter." Similar observations have been 

 made in other countries. For this reason they have 

 to be generally wintered in frames in the Botanic 

 Gardens at Kew and Dublin, and are thus exposed to 

 higher temperatures than at present obtain in the 

 British Islands. This fact suggests that the Alpine 

 and Arctic plants really did not originate in countries 

 with cold temperatures. They probably made their 

 first appearance long before the Glacial period 



