2QO THE BIOLOGY OF THE SEASONS 



half-century. In many cases it became impossible to return 

 to the old home in Spring. The whole of Britain, for in- 

 stance, was probably ice-covered, except a narrow stretch 

 along the South of England. Those birds that could not 

 endure the more southerly breeding and brooding would 

 be eliminated, those that were slow to recognise the menace 

 of approaching Winter would be eliminated, and those that 

 had no sense of direction would be eliminated, and the 

 migratory instinct would diffuse itself and become more and 

 more precise. 



After the Ages of Horror were passed and the ice had 

 in great part retreated to the Polar Regions, there came 

 about what we may still see continuing the recolonisation 

 of the North Temperate Zone as a breeding area. At the 

 present time there are some birds, such as starlings, which 

 are pressing farther and farther northward year after year. 

 In connection with this return to old racial haunts which the 

 Ice Age had rendered quite unhabitable, we must remember 

 that other climatic changes were probably in progress 

 which made the South a more and more difficult region 

 for secure breeding and successful brooding. 



In general terms, then, the present-day Spring migra- 

 tion northwards implies an organic reminiscence of the 

 original headquarters before the Ice Ages ; and the 

 present-day Autumn migration southwards implies an 

 organic reminiscence of the second home which was dis- 

 covered under the stress of the glacial invasion. And 

 the paths of the migrants to-day may still correspond in 

 some measure to those established by slow degrees as the 

 ice flowed and ebbed. It is certainly very significant 

 that many birds from the Continent should cross to the 

 South of England in Autumn and then curve farther south- 

 wards in this way visiting the tract of Britain which was 

 not covered by the ice, 



