THE PJIILOSOPHY OF MIGRATION. 71 



repeatedly by Gatke whilst studying migration for 

 half a century on the tiny island of Heligoland, 

 one of the best adapted spots in all the world for 

 the purpose. The order of return in the following 

 spring is partially reversed. Then the adult males 

 take precedence; the females travel next in order; 

 the birds of the year follow these, and last, as usual, 

 come the weakly and the w^ounded, following on 

 in the wake of their more vigorous and healthy 

 comrades, like so many camp-followers and disabled 

 travellers in the rear of a marching army ! 



Tiie daily time of migration flight varies a good 

 deal. Some species more habitually migrate by 

 day, and may frequently be watched on their journey 

 north or south, as the case may be, all the time that 

 the sun is above the horizon. The great majoritv 

 of birds, however, migrate by night, or if they do 

 pass by day, it is above the range of human vision. 

 Many species, however, will continue their flight 

 along certain routes after the sun has risen ; yet, on 

 the other hand, numbers prefer to rest for the day, 

 provided they are on land, wherever they may 

 chance to be, passing on again with the recurrence 

 of darkness. Every observer of birds must have 

 remarked these interesting facts over and over 

 again. At sunset one day not a bird of a certain 

 species can be found anywhere ; at dawn the next 

 the place may swarm with them. On the other 

 hand, he may see a species abundantly one evening, 

 and search for it in vain by daylight on the follow- 

 ing morn. During spring and autumn, odd birds, 



