22 DISPERSAL AND MIGRATION. [part i. 



but it nevertheless breeds in the Jordan Valley, so that in 

 some places it is only the surplus population that migrates. 

 In August and September, all who can return to their winter 

 quarters. 



Migrations of this type probably date back from at least the 

 period when there was continuous land along the route passed 

 over; and it is a suggestive fact that this land connection is 

 known to have existed in recent geological times. Britain was 

 connected with the Continent during, and probably before, the 

 glacial epoch ; and Gibraltar, as well as Sicily and Malta, were 

 also recently united with Africa, as is proved by the fossil 

 elephants and other large mammalia found in their caverns, by 

 the comparatively shallow water still existing in this part of the 

 Mediterranean while the remainder is of oceanic profundity, 

 and by the large amount of identity in the species of land animals 

 still inhabiting the opposite shores of the Mediterranean. The 

 submersion of these two tracts of land (which were perhaps of 

 considerable extent) would be a slow process, and from year to 

 year the change might be hardly perceptible. It is easy to see 

 how the migration that had once taken place over continuous 

 land would be kept up, first over lagoons and marshes, then over 

 a narrow channel, and subsequently over a considerable sea, 

 no one generation of birds ever perceiving any difference in the 

 route. 



There is, however, no doubt that the sea-passage is now very 

 dangerous to many birds. Quails cross in immense flocks, and 

 great numbers are drowned at sea whenever the weather is un- 

 favourable. Some individuals always stay through the winter 

 in the south of Europe, and a few even in England and Ireland ; 

 and were the sea to become a little wider the migration would 

 cease, and the quail, like some other birds, would remain 

 divided between south Europe and north Africa. Aquatic 

 birds are observed to follow the routes of great rivers and 

 lakes, and the shores of the sea. One great body reaches 

 central Europe by way of the Danube from the shores of the 

 Black Sea; another ascends the Rhone Valley from the Gulf 

 of Lyons. 



