THE GLACIAL RAXGE CONTRACTION, ETC. 131 



keeping out the warm currents, and the English Channel 

 and North Sea being then a continuous land mass.^ With 

 the return of the more genial climate birds soon began 

 to emigrate from the southern Refuge Areas (I. and II.), 

 following the retreating ice and snow-fields north ; so 

 that by the time the climate of the North Sea Plains 

 and West Continental Europe was sufficiently genial, a 

 dominant resident avifauna chiefly composed of hardy 

 species was already established in our area. With the 

 growth of vegetation on this prehistoric plain and in 

 West Europe the birds began to extend (or in other 

 words to emigrate) their range east across that area. 

 The climate, however, was too severe in winter to allow 

 of these birds becoming resident ; they merely migrated 

 in spring further and further east each century to breed, 

 coming back in autumn to winter in the mild climate of 

 the Gulf Stream laved west. Conditions favourable to 

 successful colonization and increase in Europe continued, 

 and these West to East migrants multiplied accordingly 

 spreading east and north-east across Europe even to 

 West Asia, yet compelled by the severer climate of the 

 north and east to return west every autumn. But 

 gradually the sea began to encroach upon the land ; the 

 plains between the Continent and our eastern borders 

 slowly became more and more water-logged as sub- 

 mergence went on, and the great central river valley 

 became perhaps a fjord, which, however, the eastern 

 migrants found no difficulty in crossing. Slowly each 

 century this ocean inlet became wider ; the water 



1 We must also not overlook the fact that the fourth Glacial 

 Period — the epoch of the Great Baltic Glacier — which did not affect 

 our area or the continental lands adjoining it to any great extent, 

 must have had a vast influence on the emigration of species. 



