The geographical distribution of animals, with a study of the relations ...

;22 DISPERSAL AND MIGRATION. [part i.but it nevertheless breeds in the Jordan Valley, so that insome places it is only the surplus population that migrates.In August and September, all who can- return to their winterquarters.Migrations of this type probably dateback from at least theperiod when there was continuous land along the routepassedover; and it is a suggestive fact that this land connection isknown to have existed in recent geological times. Britain wasconnected with the Continent during,and probably before, theglacial epoch ; and Gibraltar, as well as Sicily and Malta, werealso recently united with Africa, as is proved by the fossilelephants and other large mammalia found in their caverns,the comparatively shallow water stillbyexisting in this part of theMediterranean while the remainder is of oceanic profundity,and by the large amount of identity in the species of land animalsstill inhabiting the opposite shores of the Mediterranean. Thesubmersion of these two tracts of land (which were perhaps ofconsiderable extent) would be a slow process, and from year toyear the change might be hardly perceptible. It is easy to seehow the migration that had once taken place over continuousland would be kept up, first over lagoons and marshes, then overa narrow channel, and subsequently over a considerable sea,no one generation of birds ever perceiving any difference in theroute.There is, however, no doubt that the sea-passage is now verydangerous, to many birds. Quails cross in immense flocks, andgreat numbers are drowned at sea whenever the weather is unfavourable.Some individuals always stay through the winterin the south of Europe, and a few even in England and Irelandand were thesea to become a little wider the migration wouldcease, and the quail, like some other birds, would remaindivided between south Europe and north Africa. Aquaticbirds are observed to follow the routes of great rivers andlakes, and the shores of the sea. One great body reachescentral Europe by way of the Danube from the shores of theBlack Sea; another ascends the Rhone Valley from the Gulfof Lyons.