22 2 THE MIGRATION OF BIRDS. 



southern Flight been performed, that the impulse 

 is decidedly an hereditary one, as is proved by 

 the fact that young birds and birds whose sexual 

 instincts have been accidentally suppressed, are 

 the first to obey its promptings and to pass 

 south in autumn, just as we have seen that 

 sexual passion being the strongest only in adults 

 sends them first to the north in spring, and that 

 in many cases (if not in all species where the 

 young do not breed in their first spring) the 

 young lag behind. The terrors of that far-off 

 Ice Age, the dismay attending the banishment 

 of Birds from the Polar world, have apparently been 

 so deeply impressed upon migrants, that they have 

 become hereditary terrors — an impulse, a restless 

 longing desire, even in the young and inex- 

 perienced, to hurry away to warmer regions at 

 the first possible moment. Curiously enough this 

 impulse appears to be the most strongly developed 

 in birds that breed the furthest north, where the 

 extremes of an Arctic climate are the most pro- 

 nounced. And what is equally remarkable is, that 

 as soon as these migrants have reached more 

 temperate latitudes they begin to loiter about, and 

 to approach their winter quarters in a more leisurely 

 way, as if conscious that they had left those regions 

 behind where sudden winter might overtake them. 

 We may thus very fairly attribute the Autumn 

 Migration of birds to a fall of temperature at 

 the approach of winter in their breeding-grounds, 

 just as we have seen that the Spring Migration 



