370 THE STRUCTURE AND LIFE OF BIRDS cuHap. 
and wheat is extended to new territories. Birds come 
wherever there is food for them. Mr. Seebohm men- 
tions that when there has been a grass fire in South 
Africa, the scene is visited by Lapwings, Coursers, 
and Pratincoles eager to pick up the burnt grass- 
hoppers. The Woodpigeon, as already mentioned, is 
attracted by the acorns and turnips of England. 
These opportunist, gipsy migrations may supply a 
clue to enable us to find the cause of the grand mi- 
grations that admit of no irregularity. Where food 
is, there are animals to eat it. On mountains above 
the level at which grass or flowers grow, the scanty 
lichens upon the rocks support small wingless insects. 
On glaciers, if you lift a stone, you will often find 
upon the ice below numbers of “glacier fleas,”? 
which seem to have nothing but the lichen on the 
stones on which they can live. Stagnant ponds 
teem with Hydras Rotifers, Amcebz, Vorticelle. 
The lowest depths of the Atlantic, where there is no 
kind of vegetable growth, are peopled with fish and 
crustaceans, supported directly or indirectly by the 
débris of animal life that descends from the surface 
waters, What wonder, then, that birds in spring are 
found hard at work upon the cranberries and crow- 
berries that in Arctic regions have remained frozen 
during the winter, or that insect-eaters are attracted by 
the countless mosquitoes? If there had been all this 
enormous supply of food and no demand, there would 
then have been a far more difficult problem. Climate 
may, no doubt, have been the cause in some cases. 
But, often, this must have acted only indirectly by 
1 Jsotoma Saltans, an apterous insect. 
