242 Canon Tristram on the Polar Origin of Life ^c. 



central mass^ — which during the Miocene period was a vast 

 shallow sea-basin, thus quite interrupting any retrocession of 

 the great previous immigration into South America in the 

 Triassic age, — had been upraised and become a vast table- 

 land drained by the feeders of the Mississippi. 



I propose to defer to another paper a more detailed conside- 

 ration of the mode of distribution of many other families, such 

 as the Thrushes, and of the insular avifauna, and especially 

 of the limits and operation of the glacial epoch, premising 

 that I am not prepared to invent glacial epochs as an easy 

 solution of difficulties. I will now only add — what I hope to 

 work out more fully hereafter — that in the Polar origin of life 

 we seem to have a key to that perplexing riddle, the migra- 

 tion of birds. 



All ornithologists are aware of the instincts, strong in all 

 species of birds, without exception, which attract them to 

 the place of their nativity. When increasing cold drove the 

 mammals southward, they could not retrace their steps, 

 because the increasing Polar sea, as the Arctic continent 

 sank, barred their way. The birds reluctantly left their 

 homes as winter came on, and followed the supply of food. 

 But as the season in their new residences became hotter in 

 summer, they instinctively returned to their birthj)laces and 

 there reared their young, retiring with them when the 

 recurring winter impelled them to seek a milder climate. 

 Those species which, unfitted for a greater amount of heat 

 by their more protracted sojourn in the northern regions, 

 persisted in revisiting their ancestral homes, or getting as 

 near to them as they could, retained a capacity for enjoying 

 a temperate climate, which would gradually be lost by the 

 species which settled down more permanently in their new 

 quarters ; and thus a law of migration became established on 

 the one side, and sedentary habits on the other. 



