430 The Outline of Science 



circumstance capable of initiating the habit which is still main- 

 tained. Some have supposed that the last Glacial Epoch, or Great 

 Ice Age, may have driven birds gradually southwards, and after 

 a long time allowed them to return gradually northwards: but 

 during the second phase, it is thought, they would come north only 

 for the summer and return in between to the alternative home they 

 had learnt to know. Others have imagined birds as originating in 

 the south and gradually extending their range in search of fresh 

 feeding-grounds for the hungry mouths of the breeding season, 

 going further and further each summer, but always returning in 

 winter to the original cradle of the race. 



If we admit that the immediate seasonal changes are insuf- 

 ficient in themselves to cause migration, beginning so early in each 

 autumn as it does, we must yet invoke them to some extent to 

 complete the other theory. If migration is an ancient habit, 

 annually reborn, there must still be some immediate factor stimu- 

 lating the latent instinct. Events not in themselves of sufficient 

 strength as causes may yet serve to release more powerful energies, 

 just as a detonator explodes the bursting-charge: so many subtle 

 changes, either in the seasons or perhaps in the functional cycle 

 of the bird's life, awaken the compelling instinct which causes 

 birds to cross unknown seas and continents in accordance with 

 some ancient plan. 



How do Migrants Find their Way? 



What routes do migrants follow, and how do the birds find 

 their way? We must remember here, again, that migration is in 

 the main a very orderly phenomenon which takes place year after 

 year according to the same pattern. We have now evidence, too, 

 that as regards summer quarters, at least, it is common for birds 

 to return to the same places with great accuracy. Any suggestion, 

 therefore, of a mere haphazard movement with a vague general 

 direction may be dismissed as being inconsistent with the facts as 

 we know them. Other points to be remembered are that much 



