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. 
