DISPERSAL AND MIGRATIONS 169 



they are able, given favourable meteorological 

 conditions, to perform the journey with the 

 utmost confidence, and even in a single, more 

 or less uninterrupted, flight. The habit of mi- 

 grating across seas was acquired when those 

 areas were dry land. There are few (we doubt 

 if there are any) or no instances of migration 

 across deep and ancient waterways ; the sea 

 has gradually replaced the land as the habit 

 of migrating across the area was formed. Not 

 only so ; at the great altitude at which birds 

 migrate there are few if any waterw^ays that 

 cannot be spanned by the eye of a bird migrat- 

 ing over them, one coast-line coming into view 

 as soon as, or even before, the other is lost. 

 We may here state that some writers have 

 asserted that certain birds reach their breeding- 

 grounds by one route and return to their winter 

 quarters by another, but there is not a shred 

 of reliable evidence to support the contention. 

 If such were the case migration would then 

 reach to a degree of mystery that even the 

 wildest theory has yet failed to carry it. 



The general aspects of Migration are full of 

 absorbing interest. Some birds appear to travel 

 exclusively by day, others as habitually by night, 

 others yet again both by day and night. The 



