44 THE MIGRATION OF BIRDS. 



breeding at or above the Arctic Circle (and doubtless 

 bred much farther north during milder climatic 

 conditions), and still possess the habit of feeding far 

 into the twilight. This, I consider, seems to imply 

 that they or their ancestors (for the habit widely 

 prevails throughout the Turdid^) continued to 

 inhabit the northern world during the Polar night, 

 so long as climatal conditions permitted a constant 

 residence in these regions both of themselves and 

 the various creatures on which they fed. That 

 want of light may have been an initiating cause of 

 Migration is not altogether improbable; but a 

 gradually lowering temperature was undoubtedly 

 the most potent^ and as applied to the Waders and 

 the Ducks the only cause. 



It has been laid down as an axiom in Ornithology 

 that the birds that go the furthest north to breed 

 go the furthest south to winter; but the vast 

 importance of this fact as bearing on the past 

 history of Migration appears hitherto to have escaped 

 notice. It is an interesting fact in itself, but to my 

 mind the cause of the habit is immeasurably more 

 interesting, indicating, I would suggest, an Ancient 

 Migration extending almost from Pole to Pole. 

 That the Antarctic continent (a circular mass of 

 land more than twice the area of Australia) during 

 remote ages has from time to time enjoyed periods 

 of climate mild enough to support a fauna and 

 flora as rich as or even much richer than those 

 now peculiar to Arctic lands, seems indisputable. 

 The botanical evidence is certainly in favour of 



