130 BIRD-LIFE OF THE BORDERS 



theless, to disbelieve that these migrations are accom- 

 plished in the way that is ordinarily accepted — that is, 

 by hard straightaway flying. There remain yet to be 

 discovered the auxiliary factors that alone can render 

 such feats possible — those natural or physical advantages, 

 or favouring conditions, not necessarily recondite, that 

 will bring the phenomenon within the radius of practic- 

 able performance. 



The anomaly puzzled ancient thinkers ages ago. 

 They suggested that larger birds carried the smaller 

 over-sea, in fact that migrating storks and cranes took 

 passengers. Nearer our own times, Gilbert White gravely 

 ruminated on the alternative of hibernation as a solvent 

 of the perplexity. Such ideas are abandoned ; but we 

 have not yet touched solid ground, and still remain in 

 the region of theory and conjecture. 



Birds are warmer-blooded than ourselves or other 

 mammalia, and are capable of sustaining life in rarified 

 atmospheres where these could not. By a simple 

 mechanical ascent, they can reach, within a league or 

 two, regions and conditions quite beyond human know- 

 ledge : where, selecting favouring air-strata, they may 

 be able to rest without exertion ; or find meteorological 

 or atmospheric forces that mitigate or abolish the labours 

 of ordinary flight, or possibly assist their progress. The 

 explanation may be simple ; some force or factor over- 

 looked, though it may be, perhaps, in full view — or 

 perhaps, at present, unknown. 



It is in the upper regions of open space where, 

 I suggest, the final clue will be found. Nowhere else 

 on this earth does there remain a region (within the 

 trifling span of a league or two) which yet resists the 

 ingenuity and the enterprise of mankind to pierce and 



