PELAGIC BIRD FAUNAS OF THE INDO-PACIFIC OCEANS 469 



In the southwest Pacific Lams novae-hollandiae is a common species 

 in southern AustraHa and New Zealand, and though found in northern 

 AustraHa it is sparse and locally absent from considerable stretches of 

 that coastline between the Lacepedes Islands in the west and central 

 Queensland in the east. It has, however, colonised New Caledonia and 

 the Chatham Islands. 



3. PAN-TROPICAL ELEMENTS. 



The present pan-tropical fauna of the three oceans— Atlantic, In- 

 dian and Pacific— is the product of a development of considerable anti- 

 quity in a comparatively uniform environment, and is now presented 

 in richest profusion in the Indo-Malayan region. Ekman (1953, pp. 

 77-79) shows that this is not due to the Indo-Malayan region being 

 the cradle of this fauna, which is basically pan-tropical and descended 

 from a more or less homogeneous Tethys fauna. Climatic and geo- 

 graphic changes in the Miocene and Pliocene dismembered the Tethys 

 fauna and it survived with least drastic change in the present Indo- 

 Malayan area. Ekman in a succinct summary (ibid., p. 79) states: 

 "The present paucity of the Atlantic fauna is not caused by its position 

 on the periphery of an Indo-Pacific centre from which it received only 

 a small part of faunal elements radiating from there. The truth is 

 really that it suffered a deterioration of climate. The faunal richness 

 of the Indo-Malayan region cannot be explained by the assumption 

 that this region became a developmental centre for whole classes and 

 orders of the animal kingdom to a greater degree than, for instance, 

 the Atlantic Ocean. The explanation is rather that in contrast to the 

 Atlantic the Indo-Malayan region has been able to preserve this in- 

 herited richness until the present time, and that in addition new forms 

 have been able to develop continuously." 



Ekman was dealing primarily with the marine invertebrates and 

 fishes of the coastal shelves, but his conclusions can well be applied 

 to the pelagic sea-birds. The present pan-tropical bird fauna contains, 

 in addition to the primary elements just referred to, a number of im- 

 migrants from, mainly, the southern cool-water fauna and which have 

 in several instances undergone secondary differentiation in tropical and 

 sub-tropical waters. 



It is convenient, therefore, to consider the Pan-tropical Fauna in 

 two groups: (a) the descendants of forms which have always apparently 

 been tropical in distribution, and (b) forms which are secondarily 

 tropical and which show evidence of descent fiom southern colonists 

 from the later Tertiary onwards. 



