52 THE MIGRATION OF BRITISH BIRDS 



summer characteristic of some of the glacial ages, and 

 enormous tracts of low-lying country were inundated, as 

 for instance in parts of Middle Europe and Southern 

 Russia, in the midst of which birds must then, as we 

 know the)' do now, have found a perfect paradise, teem- 

 ing with food. Similar floods, but on nothing like so 

 gigantic a scale, caused by the rapid melting of vast 

 quantities of snow and ice, continue to be one of the 

 most characteristic features of the continental Arctic 

 regions, especially in the river valleys, where the break- 

 up of the streams in the scarcely-perceptible northern 

 spring is the grandest natural phenomenon of each 

 recurring year. The melting of the snow from the 

 Alps, causing vast areas to be annually flooded, is 

 another present day instance. The higher latitudes of 

 course, then as now, were subject to the greatest cold. 

 The range of birds throughout the varying phases of the 

 Ice Age must therefore have expanded and contracted 

 north and south as the climate alternately favoured or 

 enforced one movement or the other — oscillated as it 

 were between northern and southern areas with each 

 mild or cold Glacial Period, It will probably ever 

 remain a hopeless task to attempt to portray those 

 avian movements, or to indicate the probable species 

 that made them. Our only chance of success seems to 

 rest with the close of the glacial era ; with the last cold 

 periods of the Ice Age which contracted (by extermin- 

 ation or restriction of northern breeding areas) avian 

 distribution southwards, and with its ultimate expansion 

 across those desolated northern areas, as the cold passed 

 away, and was succeeded by milder climatal conditions. 

 Broadly speaking, the last range contraction south and 



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