THE INSURGENCE OF LIFE 173 



evidence of a more direct north to south movement in 

 autumn. Large numbers of swallows are seen in autumn 

 making their way down the west coast of Africa, perhaps 

 reaching the Cape ; those from Eastern Europe are said 

 to work their way southwards by the Nile Valley. Cor- 

 responding species or varieties in North America seem to 

 fly to Brazil, and in North Asia to Burmah. 



It is not merely in regard to the routes followed by 

 migratory birds that we are in ignorance ; we are in most 

 cases quite unable to say where our summer visitors pass 

 the winter. We know that they leave us for the south, 

 and we know that birds of that kind become numerous in 

 the late autumn in some other area the shores of 

 the Mediterranean, Arabia, West Africa, South Africa 

 and so on, but what we wish to be able to do is to make 

 a precise statement to the effect that certain summer 

 visitors of the Midlands of England spend their winter on 

 the Gold Coast or elsewhere. Perhaps this will eventually 

 become possible if the bird-marking method is prosecuted 

 for a long stretch of years. Another question of great 

 interest, which must wait for its answer until many more 

 data accumulate, is whether the return- journey in spring 

 is by a route different from that of the autumnal journey. 



Other matters for investigation, which must be patiently 

 continued without hurrying towards an answer, are the 

 altitude and the velocity of the migratory flight, and its 

 relation to weather-conditions. While enormous armies of 

 larks, starlings, thrushes, and some other birds have been 

 seen flying very low across the sea, it is probable that 

 most migrants fly at a considerable height. Careful 

 observations made by von Lucanus lead to the conclusion 

 that it is very unusual for birds to migrate at altitudes above 



