344 Wonders of the Bird World 



Before the morning's light every bird has gone. This may, 

 perhaps, be explained by the assumption that migration is 

 performed at altitudes determined by the height at which 

 a favourite air-current is found ; that, in this case, the air- 

 current had ceased to flow, and (no other being found within 

 available limits) the migration in question was necessarily 

 suspended, awaiting a renewal of the required conditions. 

 Fortunate birds to have the land beneath them. Should 

 such suspension occur at sea, it would involve the destruc- 

 tion of the entire host. 



" Except by aid derived from operation of physical laws, 

 the nature and extent of which are unknown to me, and 

 by taking advantage of ' trade wind ' circulations in the 

 upper air, I believe that migration is impossible for short- 

 winged forms of sedentary habit, such as those above- 

 named. But that aid, and those advantages, may vastly 

 facilitate, and perhaps vastly accelerate, a process which is 

 otherwise impossible. As to insects, there appears no 

 evidence to prove that there exists any regular migration 

 across the North Sea. Occasional irruptions such as those 

 of Colias edusa and of the Diamond -back moth may be 

 thus explained. But in most cases, I take it, insects seen 

 in mid-sea are probably blown-away bands that, during 

 some local inland migration, have been caught in an air- 

 stratum too strong for their power of flight. Few very 

 few of these will survive to cross the sea." 



Experiments made by Mr. Frank M. Chapman in 

 America, with a powerful telescope directed towards the 

 moon, have demonstrated the fact that birds can be seen 

 passing at night at an altitude of from one to five miles, 

 and it was even thought that some of the species could 

 be recognized, though the greater number were naturally 

 indistinguishable. 



It is, therefore, certain that a great deal remains to be 

 done before we can pretend to know much about the 



