37o THE STRUCTURE AND LIFE OF BIRDS chap. 



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," x 

 which seem to have nothing but the lichen on the 

 stones on which they can live. Stagnant ponds 

 teem with Hydras Rotifers, Amcebae, Vorticellae. 

 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 

 debris 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 Isotoma Salta?is y an apterous insect. 



