198 THE MIGRATION OF BIRDS. 



In South Africa the northern zone of winter 

 migration extends through the Transvaal and 

 Damara Land to the Congo and the great system 

 of equatorial lakes at the source of the Nile. In 

 South America the northern destination of the 

 Neotropical migrants reaches through La Plata to 

 Peru and Brazil ; and here, but on a much smaller 

 scale, we have a sea flight north from the Falklands 

 and Tierra del Fuego. Precisely the same causes 

 influence migration in the Southern Hemisphere as 

 in the Northern Hemisphere, scarcity of food owing 

 to a prevailing lower temperature ; although 

 nowhere is this as acute as in the Northern 

 Hemisphere. 



With the return of spring in either Hemisphere 

 the great wave of bird-life once more begins to 

 flow north or south, according to circumstances, to 

 the breeding-grounds in more temperate climes — a 

 vast tide of Migrants that permeates every district 

 and only spends itself in regions as far north or 

 south as land is known ! The great variation in 

 climate over so vast an area of course determines the 

 migration limits of individual species, due regard 

 being paid to the isothermal lines of temperature. 

 I am of opinion that much light will yet be thrown 

 on the Phenomenon of Migration generally when 

 this important condition is more closely studied. 

 That Temperature has a great eflfect on a choice of 

 breeding-grounds is unquestionable ; that Temper- 

 ature varies greatly over wide areas in obedience to 

 ocean currents and other influences irrespective of 



