298 THE MIGRATION OF BRITISH BIRDS 



on, from where or whence did the " Arctic" flora arrive ? 

 True, many of the species — such as the dwarf birch for 

 instance — are but modified forms of more Temperate 

 types, and are the result of Post-Glacial invasion from 

 southern bases ; but, paradoxical as it may appear, the 

 strictly Arctic or Inter-polar flora could never have been 

 developed within the Polar regions of either hemisphere. 

 We are forced to the conclusion that it must have had 

 an origin in a region where cold conditions have existed 

 unchanged for a vast and indefinite period of time. No 

 part of the world fulfils these conditions except the 

 mountains and highlands within the tropics. Here we 

 can conceive how vegetation slowly crept up these 

 heights from the equatorial plains, becoming modified 

 as it reached those zones where cool climates prevailed 

 — how it spread in strict accordance with the Law of 

 Dispersal north or south along the mountain chains 

 towards the poles, entering the Arctic or Antarctic 

 regions, and becoming dominant as they offered favour- 

 able conditions for its increase, the altitudinal range 

 becoming lower as the latitudinal range became higher. 

 We can also understand how when a change to a warm 

 climate occurred (a mild inter-glacial period) this flora 

 dwindled away and perished in these Polar latitudes, 

 and was only preserved on the higher and more southern 

 range bases ; or in like manner how when the Glacial 

 Epoch assumed an intense phase it became exterminated 

 and buried under the ice-sheets and snow-fields, but 

 maintained its existence through the southern range 

 bases which preserved those portions of the species that 

 dwelt in areas beyond glacial influence, supplying fresh 

 colonists to emigrate towards the glaciated regions, as 



