Canon Tristram on the Polar Origin of Life. 213 



specifically or subspecifically distinct from long isolation : 

 Java_, New Guinea^ Timor Laut, N. Australia, S. Australia, 

 Tasmania,, and probably other islands, possessing their re- 

 cognized sedentary varieties, but all mountaineers in the 

 breeding-season. 



Next, but long after, when the family within the Arctic 

 continent had learned in some degree to adapt itself to the 

 diminishing temperature, and had, at the same time, modified 

 its adult plumage, the great mass of the Thrushes were 

 driven southward on the second or great migration. The 

 retreating parties would seem to have followed all the possible 

 lines of retreat, and many of them to have boldly crossed the 

 ocean north of Siberia and followed up the course of its mighty 

 rivers. Many remained in the north, some of whom, repre- 

 sented by our Fieldfares and Redwings, clung most perti- 

 naciously to their homes, and went no further than compelled 

 by dire necessity. Others, among them the ancestors of our 

 Song Thrush and Missel Thrush, adopted various routes, 

 but principally the East Atlantic and the Siberian rivers, 

 and returning north each year, have, by mutual intercourse, 

 maintained across Europe and the greater part of Asia the 

 ancestral type unchanged. Again, a considerable portion 

 of these migrants, following down the west of Africa, 

 spread eastward over that continent, and, their return being 

 barred by the Saharan desert, became strictly sedentary in 

 their various localities, where they have become differ- 

 entiated into some dozen species. A straggler or two of 

 this adventurous flight reached even Tristan d^Acunha, Avhere 

 his Avings were so clipped that he abandoned all thought of 

 foreign travel ; until now some of his friends refuse to recog- 

 nize him, and would have us believe that he is no Thrush, 

 but a Timaline. 



, The small size, the spotted breast, the strong generic afii- 

 nities of the Nearctic Thrushes, and their marked distinction 

 from all the South-American species, would lead me to 

 believe that they are descendants of ancestors which peopled 

 N. America by way of Greenland and Labrador : probably at 

 a period when the central area was submerged. They have 



