ECOLOGICAL 75 



the broad fact that an almost universal problem in the North is 

 how to meet the winter. Some creatures lay up stores and others 

 put on fat ; some sink into coma and others hibernate ; some increase 

 the thickness of their coat and others put on whiteness. But the 

 neatest of all the solutions of the winter problem is migration — 

 circumventing the winter entirely. 



Not to be forgotten is the tendency that many animals have to 

 scatter or disperse from territories that become overcrowded after 

 the numbers have been increased by breeding. And another hint is 

 to be found in the natural history rule that a new generation is 

 cradled in the ancestral haunt of the race. The land-crab gees far up 

 the hills, but it comes back to the sea to spawn. Open sea turtles, 

 which had terrestrial ancestors, come back to the sandy shores to 

 lay their eggs. So in other cases and so with the migrant birds, the 

 cradle is in the ancestral home. 



The fifth problem is that of way-finding. No doubt there is much 

 mortality in migration. Some migrants lose their way on the path- 

 less sea and perish of hunger; some are driven by storms far from 

 their normal course; some are fatally attracted to lighthouses and 

 dash themselves against the windows. And yet the larger fact is 

 that the migration is usually successful; the travellers reach their 

 winter quarters, and ringing experiments prove that they sometimes 

 return year after year to the place of their birth. How do they find 

 their way? 



It must be allowed that birds sometimes utilise landmarks — 

 mountain-ranges, river-valleys, chains of islands. Visual acuteness 

 often counts for something. Fogs are well known to be much against 

 success. Carrier-pigeons, so clever at their geography, get their early 

 lessons by day. Yet keen sight cannot be more than part of the 

 answer, since many birds migrate in the dark and cross great tracts 

 of ocean where there are no landmarks. It has been shown that terns 

 transported in closed baskets on board ship into unknown waters 

 may sometimes return within a few days to their nests, even from 

 a distance of 800-1,000 miles. But we must confess that we do not 

 know how it is done. 



Some naturaUsts still adhere to the theory that the success 

 attending migration must be due to the cumulative inheritance of 

 the results of experience. But it is not certain that this kind of 

 hereditary entailment is ever possible ; and it is difficult to say what 

 sort of experience could be gained by birds that migrate in the 

 darkness at great heights and over the trackless sea. 



Nor can we lay much stress on the tradition theory, that those 

 birds will lead well in 1930, who followed well in 1929 and 1928. 

 For the young birds often leave before their elders. 



Thus we seem almost compelled at present to suppose that birds 

 are in high degree hereditarily endowed with a sense of direction. 



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