THE rillLOSOrilY OF MIGRATION. 79 



of Cranes being observed on passage at a great 

 elevation crossing over the Pamir Plateau in Central 

 Asia, a district which is upwards of 16,000 feet 

 above the level of the sea. Much evidence might 

 also be given of migration extending through lofty 

 mountain passes, which it is only fair to presume 

 are reached by horizontal flight, rather than by 

 vertical flight from the plains below. Some of the 

 most elevated land (not actually mountain summits) 

 in the whole world is known to be the pathway of 

 migratory birds. 



By far the most important benefit derived from 

 following an extreme lofty course is undoubtedly 

 that of increased range of vision. The higher a 

 bird flies the further it can see, the more extended 

 becomes the visible segment of the earth's spliere 

 below it. It is probably almost entirely due to its 

 aerial existence that a bird's powers of perception 

 and knowledge of locality are so acutely developed. 

 There can be little doubt that the habit of con- 

 stantly viewing the country at various heights, in 

 all kinds of weather, and under ever- changing at- 

 mospheric influence, endows a bird with a know- 

 ledge of topography that seems to a terrestrial 

 animal like man as little short of marvellous. The 

 lay of the country can be seen at a glance from an 

 aerial point of observation ; and it is this well-known 

 fact that leads a hunter lost in the forest to cUmb 

 the nearest tree to ascertain his bearings. Each 

 migratory bird must have a wonderful knowledge 

 of the topography of its own particular routes, aided 



