178 THE WONDER OF LIFE 



augmentation and specialization of the migratory instinct 

 on ordinary Darwinian lines. Discriminate elimination 

 of the dull, the sluggish, the wilful, the inexpert would 

 gradually raise the standard of migratory capacity mil- 

 lennium after millennium. 



As to the actual historical conditions that justified the 

 migration experiment and sustained the discriminate 

 eUmination of the inexpert, there are two theories, both 

 of which may be true. On one theory, our present-day sum- 

 mer visitors were once at home over a great part of the 

 Northern Hemisphere which once had a much warmer and 

 more equable climate than it now enjoys. Then there 

 was no need for much migration, though most of the 

 birds woidd probably seek to get away from the warmer 

 areas at the breeding and brooding time, and away from 

 the more exposed northern outposts when winter came. 

 But if the climate changed and became steadily more 

 severe, if the winters lengthened and the snowline crept 

 lower and lower down on the mountains, if great glaciers 

 spread southwards, and so on, then very gradually birds 

 had to migrate further and further south in winter and were 

 able to penetrate less and less far into the north in spring. 

 When the chmate changed again for the better, and the 

 ice retreated pole-wards, there came about a re-coloniza- 

 tion of the North Temperate zone as a breeding area. There 

 was a return to the old racial haunts which the Ice Ages had 

 rendered temporarily uninhabitable. 



In general terms, then, the present- day spring migration 

 northwards imphes an organic reminiscence of the original 

 headquarters before the Ice Ages ; and the present-day 

 autumn migration southwards implies an organic remini- 

 scence of the second home which was discovered under the 



