i68 THE STORY OF THE BIRDS 



naturalist suggested that birds found their way 

 across seas by noticing the peculiar roll of 

 the waves ; but these are constantly varying in 

 direction, especially in narrow land-locked seas 

 which are crossed by migrants, according to 

 the influence of ever - changing winds. The 

 reader may perfectly well rest assured that all 

 this theorising is profound nonsense. The 

 theory of instinct may be very pretty and very 

 attractive, but it is not supported by logic or 

 by facts. Birds migrate purely and simply by 

 an exercise of those extremely acute powers of 

 observation and keen aptitude for recognising 

 landmarks, the result of an intensely receptive 

 memory, the result of experience, and coupled 

 with a great knowledge of locality. But these 

 powers, great as they undoubtedly are, would 

 be useless along a strange road, so that birds 

 individually keep close to certain routes, these 

 latter having been gradually formed as the 

 range has spread out from the original centre 

 of dispersal. Little by little during the history 

 of the species has the journey become longer 

 as the breeding-places became more and more 

 distant from the winter range base. The road 

 is so familiar even to those individuals that 

 breed the farthest from the winter base that 



