478 EIGHTH PACIFIC SCIENCE CONGRESS 



Reference was also made to the widespread southern Lams dominicanus 

 as a trans-equatorial transgressor of either L. marinns or L. fiisciis-ar- 

 gentatus ancestry. 



SKUAS 



Catliaracta skua 

 Hamilton (1934, p. 163) has placed the northern cold-water and 

 southern cold-water great skuas as members of one species, a taxonomic 

 treatment supported by Murphy (1936, p. 1006). Though the breeding 

 ranges of the two groups are now widely separated geographically, the 

 Chilean Skua (C. s. chilensis) migrates along the western American 

 shoreline to Alaskan waters. The group is probably of northern origin 

 and the southern seas may have been occupied by a colonisation along 

 the east Pacific sea-board, though the northern form, S. c. skua, does 

 not now occur in the north Pacific, 



GANNETS 



The Sula bassana complex 



The true classification of the ancestor of the three related temperate- 

 vv^ater Gannets, Sula hassana, S. serrator and S. capensis, is uncertain. 

 It may be a secondary immigrant into the north from the Pan-tropical 

 Fauna. 



On the assumption that the group has had its proximate origin 

 in the northern Atlantic the present geographical pattern may have 

 developed as follows: 



Sula hassana transgressed through the tropical belt of the Atlantic 

 during a period of global cooling and established itself in southern 

 Africa, where capensis became differentiated and colonised eastwards 

 to southern Australia and New Zealand, evolving there into the similar 

 race, serrator. This colonisation pattern has also been postulated by 

 Falla (1953, p. 44). 



5. SUMMARY REVIEW. 



The general picture presented by the pelagic bird faunas of the 

 Indo-Pacific basins is that in the whole of the Indian Ocean and the 

 western and central part of the Pacific we are dealing with one rather 

 homogeneous fauna which consists of the long-established descendants 

 of the Tethyan fauna of the Tertiary. Into this fauna have been in- 

 jected, from the later Tertiary onward, immigrants from the cool-water 

 faunas of the north and south, but predominantly from the south. 

 These immigrants occur in the Indian Ocean to a much less degree than 

 in the Pacific, in which ocean they are also much more numerous in 

 the south Pacific than they are north of the Equator. 



