BIRD-MIGRATION 5 



be inferred that the climate in these high latitudes 

 was once tropical. Suppose the earth, once a mass 

 of molten mineral too fervid to sustain organic life, 

 gradually cooling down, it would be at the poles 

 where the temperature would first become endur- 

 able by living creatures. Through long aeons the 

 cooling process went on, till, at last, very few forms 

 of life could endure the winter cold in a region 

 where once tree-ferns towered and giant mosses 

 steamed. Winged creatures were driven forth, fol- 

 lowing the organisms on which they depend for 

 food ; yet to this day the birds, drawn by a heredi- 

 tary impulse, press as far as possible northwards, 

 towards the land of their origin, to rear their 

 young, bearing witness that in Polar, not in 

 Equatorial regions, lies the source of animated 

 nature. 



To understand the full force of this teaching, one 

 must take extreme instances of migration, such as 

 the knot and the curlew-sandpiper afford. These 

 little birds pass so far to the north to breed that 

 it is doubtful if any human eye has ever witnessed 

 their domestic arrangements. More than sixty years 

 ago, indeed, the knot was found in company with 

 its newly hatched young on the Parry Islands, 



