PELAGIC BIRD FAUNAS OF THE INDO-PACIFIC OCEANS 475 



(p. 157) that the northern cold-water fauna is considerably richer in 

 the North Pacific than in the North Atlantic, and the explanation is 

 seen in diiferential climatic conditions, that "during the whole of the 

 Tertiary Period the North Pacific offered much more favourable con- 

 ditions for the development of a fauna adapted to a cold-temperate 

 climate than the North Atlantic" (ibid., p. 164). Milder climatic con- 

 ditions in the pre- and inter-glacial periods provided communications 

 between the two oceans. 



AUKS 



Family Alcidae 



The headquarters of the Auks are the circumpoiar northern seas, 

 and various species penetrate some distance down both coasts of the 

 north Pacific and north Atlantic Oceans. The north Pacific is par- 

 ticularly rich in species, but only one of them has succeeded in extend- 

 ing any considerable distance south. None, however, has paralleled 

 the performance of many of the southern petrels in transgressing the 

 equatorial belt into the cooler waters of the opposite hemisphere. 



The Pacific Auks have colonised the American coast southwards 

 more successfully than they have managed the Asiatic shoreline. Along 

 the American coast 5 or 6 species have penetrated as breeding species 

 to California. These are: Lunda cirrhata, Uria aalge, Cepphus colum- 

 bn, Aethia aleutica, and Brachyramphus hypoleucus. The last-men- 

 tioned has differentiated a form, B. craveri, breeding in the Gulf of 

 California, which is the only Auk to reach the Tropics. 



This distribution pattern of the auks, with a restricted southward 

 penetration on the west side of the Pacific in comparison with the 

 eastern side, is related to the hydrological pattern of the ocean. This 

 is, approximately, a mirror-image of the hydrobiological situation in the 

 South Pacific. 



PETRELS 



The Puffinus puffmus complex 

 Whatever its ultimate origin from a southern ancestry, the spread 

 of this species group in the immediate past has been as a member of the 

 northern fauna. The stimulating analysis by Murphy (1952) would 

 suggest that the centre of dispersal was the eastern Atlantic. A double 

 invasion through the Panama water gap appears to have taken place, 

 first by black-backed forms whose Pacific derivatives are P. p. auricularis 

 (breeding off western Mexico) and P. p. newelli (breeding in Hawaii). 

 The second invasion was by brown-backed birds, wliose Pacific off- 

 shoot is P. p. opisthomelas. 



