DEEPER PROBLEMS OF MIGRATION 289 



would not take hard times lying down, who was sensitive, 

 alert, restless, unconventional, adventurous, and original, 

 who was a genius, in short, a Columbus-bird. 



To meet a difficulty such as hard Winters there may 

 be detailed readjustments of what has been already estab- 

 lished ; the plumage may turn white, for instance, which 

 always helps, or a thick layer of fat may accumulate be- 

 neath the skin. But there is another way of meeting a diffi- 

 culty, by evading it altogether, and that requires genius, 

 and that was how migration was started by a number of 

 highly original birds, who discovered that the prison-doors 

 were open, and who thought it was worth while trying 

 whether Land's End in Cornwall or elsewhere was really 

 Land's End. 



All living creatures tend to flee from the uncomfortable, 

 and it is evident that, as the earth and the bird are consti- 

 tuted, things are against permanent residence in one place. 

 Moreover, in the course of the evolution of climates, since 

 birds began to be, there have been successive glacial 

 periods in which the Northern Hemisphere was certainly 

 not a fit winter-residence for birds, and for long ages not 

 a fit home for birds at all. 



The greater part of the Northern Hemisphere once had 

 a much warmer and more equable climate than it now 

 enjoys. There were magnolias blooming in Greenland and 

 palms flourishing in England as they still do in mild places 

 like Penzance. It was perhaps relatively unimportant to 

 birds where they went, though they would tend to get 

 away from the hotter regions at the breeding and brooding 

 time, and would move southward from the more exposed 

 northern outposts when Winter came. But as the climate 

 became gradually more severe, and the snowline crept 

 lower and lower on the mountains, and great glaciers were 

 formed, birds had to move farther and farther south every 



