ROUTES OF MIGRATION 225 



migration is apparent, whilst in others it is palpable even 

 to the most cursory observation. This is perfectly true, 

 and is the result of the survival of the fittest, the easiest 

 routes being occupied season by season, by the descen- 

 dants of birds that extended their area along them ; the 

 more dangerous routes being abandoned, because the 

 perils in following them have been so much greater, and 

 resulted in the gradual extermination of the individuals 

 and their descendants that had followed them during 

 their Post-Glacial emigrations. We may safely presume 

 that in earlier ages migration was most numerous 

 and important across certain areas (the mouth of the 

 English Channel, and to the south and south-west of 

 Ireland, when dry land) where it is now least appar- 

 ent and least extensive, seeing that the areas to which 

 those more ancient routes led were probably colonized 

 much earlier and in greater abundance by emigrating 

 or range-expanding species directly after the Ice Age, 

 than more northern and eastern areas, say where the 

 English Channel is now the narrowest. As the sea has 

 encroached, the migration route has become more peril- 

 ous, with the inevitable result that birds have been 

 exterminated in their persistent efforts to follow it. 

 At the present time the most used routes are those 

 where the passage is attended by the minimum of 

 danger, and where the conditions of flight are the most 

 favourable. But this is not choice on the part of the 

 individuals following these routes, but the accident of 

 direction of Post-Glacial emigration, the survival of the 

 fittest. We can thus see why the narrowest seas are 

 crossed in greatest numbers by migratory birds, why 

 the most favoured valleys, coast-lines, or mountain 



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