22 BIRD-LIFE OF THE BORDERS 



There is abundant evidence of tropical periods at the 

 Pole ; but no trace of glacial conditions in the tropics, 

 nor indeed further south than the Continent of Europe. 

 The Arctic regions have extended as far southwards 

 as the Pyrenees (where reindeer at one period existed), 

 but not much, if at all, beyond. A Polar variation to 

 that extent is explained by the phenomenon known as 

 the "nutation of the earth" — that is, the oscillation 

 of its axis accordingly as the attraction of the sun and 

 the counter-attractions of various other planets alter- 

 nately predominate. Beyond these limits (and the alter- 

 nations each occupy very many thousands of years), 

 the position of the axis appears to have been stationary 

 — that is, it has not altered to a degree which would 

 be destructive to the theory of the Polar origin of life. 



Granting, then, the substantial accuracy of what I 

 have feebly attempted to describe, it follows that the 

 North Polar regions would be the first spot on the globe 

 adapted to sustain life ; that they were, at first, the cradle 

 of all life ; and afterwards, as the Polar cold gradually 

 intensified, the centre of dispersal whence the various 

 forms were distributed throughout the world, as its various 

 portions in turn became adapted to their requirements. 



Viewed in this light, the great migratory tendency 

 towards the north becomes explicable and compre- 

 hensive enough. It simply arises from a perennial 

 instinct, which continues to draw vast numbers of the 

 feathered tribes towards the point which was originally 

 the universal home of all. It is an invariable rule that 

 all birds do breed at the most northerly points of their 

 annual range. In the northern hemisphere, the ten- 

 dency to move northwards in spring is all but universal ; 

 and, as already pointed out, there are, in many cases, 



