34 SEA-BIRDS 



that an important part of the adaptive radiation of this order was 

 comparatively late. One of the early auks, the Pliocene Mancalla of 

 California, out-penguined the great auk, Alca [Pinguinus) impennis, for it 

 had progressed far beyond it in the development of a swimming wing. 



According to Howard (1950) a few living species of birds have 

 been recorded from the Upper Pliocene, but large numbers of modern 

 forms occurred in the Pleistocene. Of course in the Pleistocene the 

 oceans approximated very closely to what they are today, with the 

 Central American land-bridge closed, the Norwegian Sea wide open 

 between Arctic and Atlantic Oceans, the Mediterranean a blind 

 diverticulum of the North Atlantic. We need this picture as a back- 

 ground to a consideration of the North Atlantic's present sea-bird 

 fauna, for wc shall find that it has few sea-bird species of its own, 

 and only two genera; for the primary sea-bird species which now 

 breed in the Atlantic (and Mediterranean) and in the neighbouring 

 parts of the Arctic, and nowhere else in the world, are no more than 

 twelve : the Manx shearwater Fuffinus puffinus* ; the very rare diablotin 

 and cahow of the West Indies and Bermuda {Pterodroma hasitata and 

 P. cahow) ; the storm-petrel Hydrobates pelagicus; the North Atlantic 

 gannet Sula bassana; the shag Phalacrocorax aristotelis; the lesser black- 

 back Larus fuscus; the great blackback L, marinus; the Mediterranean 

 gulls L. melanocephalus and L, audouinii; the Sandwich tern Thalasseus 

 sandvicensis; the razorbill Alca torda, the puffin Fratercula arctica; besides 

 the extinct Alca impennis, the great auk. The two present genera 

 peculiar to the North-Atlantic-Arctic are Hydrobates and Alca, 



The sea-birds which qualify by birth and residence to be members 

 of the North Atlantic fauna (excluding purely Arctic and Mediter- 

 ranean species) include thirteen tubenoses, seventeen cormorant- 

 pelicans, fourteen gulls, nineteen terns, two skimmers, four skuas and 

 five auks (besides various secondary sea-birds, notably about eighteen 

 ducks, three divers and two phalaropes). If we are to understand 

 how these have got into the North Atlantic we should analyse the 

 present distribution of the sea-bird orders and groups as between the 

 different oceans. 



The most primitive group of sea-birds, yet the most specialized, 

 is that of the penguins. The Sphenisci have fifteen species in all, 



* Murphy (1952) has recently united as races of this species several Australasian 

 and Pacific forms hitherto considered full species, and has suggested that the species 

 may date from the Oligocene period when the Mediterranean communicated 

 directly with the Indian Ocean. 



