148 THE WORLD OF LIFE CHAP. 



clouds of mosquitoes and their larvae that we owe the 

 very existence of a considerable proportion of the bird-life 

 in the northern hemisphere. 



The Origin of Bird-migration 



These vast Arctic plains even in Tertiary times when 

 climates were milder, would, owing to the long winter nights, 

 have always been snow-covered during several months in 

 winter although its melting might have been earlier and the 

 summer somewhat longer ; there can be little doubt that 

 the short summer with its perpetual sunshine was equally 

 favourable to the production of a superabundance of vege- 

 table and insect food very similar to what now exists there, 

 and in this fact, we find a very complete explanation 

 of how bird -migration came about Abundance of food 

 suitable for both parents and young at the season of breed- 

 ing would inevitably attract birds of all kinds from more 

 southern lands, especially as the whole area would necessarily 

 have no permanent residents or very few, but would, each 

 recurring season, be an altogether new and unoccupied but 

 most fertile country, to be reached, from any part of the 

 north temperate lands, by merely following up the melting 

 snow. And as, a few months later, the myriads of young 

 birds in addition to their parents were driven south by the 

 oncoming of the cold and darkness, they would find it 

 necessary to travel farther and farther southward, and 

 would again find their way north when the proper season 

 arrived. There would always be a considerable number 

 of the old and experienced birds to show the way ; and 

 as, with increasing severity of the seasons, the area of the 

 snow-covered plains would extend, and their capacity for 

 feeding both old and young would be increased ; there 

 would at last be brought about that marvellous rush of 

 the migrating flocks which Mr. Seebohm has so vividly 

 described. 



Before quitting the subject of migration, on which Mr. 

 Seebohm's observations throw so much light, I will shortly 

 describe the most wonderful exhibition of migration pheno- 

 mena in the world that of the small island of Heligoland, 

 40 miles off the mouth of the Elbe in about the same 



