EVOLUTION 43 



in Iceland. Honing and Salomonsen (1941) have already used the 

 English name Greenland Gull to describe it, and regard it as a race of 

 Lams argentatus. We commend to our readers, and to the compilers 

 of the Lists of the American and British Ornithologists' Union : 



the Greenland Herring-Gull, 

 Lams argentatus glaucoides {= L. a. leucopterus) 



Among the North Atlantic sea-birds are others whose species have 

 differentiated geographically and whose range-end populations have 

 become different enough to occupy the same geographical area — 

 but separate ecological niches, and thus preserve their identity. For 

 instance, it is probable that the ring-billed gull Larus delawarensis, of 

 North America, and the common gull of the Old World, L. canus, 

 have not long since shared a common ancestor, though a subspecies 

 of the common gull, which has probably spread across the Bering 

 Straits from the Old World, now occupies Alaska and parts of the 

 Canadian North- West, where it overlaps with the western element 

 of the ring-billed gull (Fig. 5). Here the two act as different species. 

 The glaucous gull and the great blackback, which overlap in eastern 

 North America, Iceland and parts of the European Arctic (Fig. 6) 

 may be not long ago descended from a common ancestor. They very 

 rarely hybridise. How the three species of terns — the arctic, common 

 and Forster's — which are very closely related, arrived at their present 

 distribution (Fig. 7, p. 46) is difficult to imagine at this stage of their 

 evolution, but they all may be descended from a common tern of 

 north-east Asia or an arctic tern of the North Pacific — from which 

 part of the world the species has probably spread, differentiated and 

 overlapped. 



Various suggestions could be made as to the origins of the two 

 guillemots, the common and Briinnich's guillemot (Fig. 8). Possibly 

 the original guillemot was a common guillemot {Uria aalge) type 

 which got divided into two subspecies in the Atlantic and Pacific 

 by the Ice Age, but not before it had had time to give rise to an arctic 

 race adapted to the harder life. After the Ice Age, with the ameliorat- 

 ing conditions, perhaps both the Atlantic and the Pacific guillemots 

 began pushing north again, this time to meet and overlap with their 

 arctic descendant, which, meantime, had differentiated sufficiently to 

 offer no direct competition. It is interesting to note that the most 

 arctic of the common guillemot races, Uria aalge hyperborea of Iceland, 

 Novaya Zemlya, and Lapland, has a very thick bill and a considerable 

 resemblance to Briinnich's guillemot, with which it, however, does not 



