FACTORS CONTROLLING MIGRATIONS 197 

 that so soon as the necessary conditions are 

 right, connection is made when the faculty, 

 telaesthesia, comes into operation, and the 

 birds, simultaneously, depart en masse. 



Some maintain that the factors controlling 

 migrations of birds are, first and foremost, the 

 wind ; and secondly, the bird's knowledge of 

 the way (landmarks ?). These say, that when 

 the wind blows cold from the north, it is a 

 warning to the birds, who then take flight and 

 are wafted in the right (southerly) direction. 

 The same when returning, the hot wind from 

 the desert brings them back ; if instinct is 

 allowed, it is only of secondary consideration. 

 This view seems hardly satisfactory, for it 

 leaves too much to chance, whereas nature 

 leaves nothing to chance. 



That wind is one of the conditions I have 

 no doubt whatever ; so are the wings of the 

 bird a condition — ^both are natural means in 

 aid of locomotion. But whether the wind 

 blows hot or cold, the bird, even if affected by 

 it, is not conscious of the fact ; if we admit 

 that, it brings us again to the reasoning mind. 

 In my belief, cold does not affect birds ; but 

 what does affect them is lack of food, which is 

 the primary cause of migration. Swallows 

 sometimes arrive in extremely cold weather 

 and, again, leave long before the cold weather 



