ENVIRONMENT, PAST AND PRESENT 89 



southward flight, evidently embodies events of 

 centuries and the experience of hundreds of genera- 

 tions of birds. Although it happens now, it is really 

 a summary of past experience. 



We must thus enquire into two quite distinct 

 things (a) Factors of the past that have induced and 

 built up the migratory custom; (b) Factors of the 

 present that annually set the migratory machinery 

 in motion. 



