BIRDS IO5 



earth could be seen from above. In that case the 

 birds at once took their direction, passed through the 

 opening, and flew downwards. 



If we compare these observations with Gatke's it 

 seems pretty certain that the birds do not need to rise 

 high during the migration-flight. It is true that the 

 question at what height they fly in clear weather is still 

 far from settled, but we may say that, if they fly at 

 a lower level in bad weather, yet probably reach their 

 end just as quickly, a high level does not seem to be 

 necessary for speed. Hence the rapidity of the 

 migration-flight is not explained by the theory of the 

 thinness of the air. 



A second question now arises. When they are at a 

 low level the birds can only see a small part of their 

 route, and none at all during the night ; how is it, then, 

 that they choose the right way at such enormous 

 distances? We will approach this question in the 

 method we have used several times already, and inquire 

 how the migratory instinct may have been developed 

 in the course of time. 



The actual migratory birds formerly lived, we assume, 

 in southern latitudes, and gradually increased until they 

 over-populated them, and food became scarce. The 

 famine was worse in the dry season, when vegetation 

 shrivels up in Africa, and the insects especially, which 

 form the chief food of the actual migratory birds, were 

 reduced in numbers. The hungry birds were thus 

 compelled to abandon their habitation, and travel in 

 every direction, seeking food and nesting-places in less 

 crowded regions. A number of them came northward ; 



