THE AUTUMN MIGRATION OF BIRDS. 237 



From the above reniarks we may conclude that 

 the xVutumn Migration of Birds is initiated by a fall 

 of temperature and consequent failure of food, in 

 conjunction with an unquestionably strong heredit- 

 ary impulse to move south from regions where ages 

 of accumulated experience have taught that a 

 winter sojourn means death. Young birds seem 

 more strongly impelled to migrate in autumn than 

 in spring, seeing that in so many species they not 

 only leave before their parents, but are more or less 

 reluctant to quit their winter quarters the following 

 spring. On the other hand, the impulse in old birds 

 to migrate in spring is stronger than in the young, 

 owing to sexual instincts being so much more 

 highly developed. The impulse to migrate in 

 young and old seems exactly reversed according 

 to season. In Autumn the Young are the eager 

 ones, the Old the laggards ; whilst in Spring the Old 

 are the eager ones, and the Young are the laggards. 

 The Migration of Birds in Autumn follows in the 

 wake of retiring summer and in the van of winter, 

 just as in spring it follows in the wake of retiring 

 winter and in the van of summer. Migration in 

 Autumn in the Southern Hemisphere, so far as we 

 can judge from the scanty details on record, is of a 

 very similar nature to that prevailing in the Northern 

 Hemisphere, and initiated by exactly the same set of 

 causes, namely, failure of food in consequence of 

 a fall of temperature. 



