PELAGIC BIRD FAUNAS OF THE INDO-PACIFIC OCEANS 465 



warding results if further studied conjointly by the ornithologist and 

 oceanographer, but it is clear from Hutchinson's treatise that more data 

 are required. 



As has been mentioned when dealing with the Bering gap, the 

 Pleistocene Ice Ages would affect the magnitudes of the various water 

 gaps. The Indo-Malayan gap, with its considerable areas of shallow 

 shelves, would have been most affected by the eustatic rise and lower- 

 ing of sea-level; Mayr has stated (1944, p. 113) that during the height 

 of the last glaciation the width of the Timor Strait, now more than 

 300 miles, was reduced to 45-75 miles. The waterways were never com- 

 pletely closed, however, during the Pleistocene and the Australian and 

 Asiatic land masses have been separated since the early Eocene. Never- 

 theless, this area of shallow seas and innumerable barrier islands, would 

 to some extent impede pelagic sea-birds and is still unoccupied by some 

 species common in the oceans on either side. 



The water gaps in the Panama zone ceased to exist by the time 

 of the Pleistocene glaciations. Hence species which made passages 

 through them, and which had earlier in their history made trans-equa- 

 torial transgressions, must have made such tropical crossings before 

 the Ice Ages. 



III. Faunal Elements in the Indo-Pacific Oceans 



1. INTRODUCTION. 



From the standpoint of their proximate origins, the pelagic sea- 

 birds may be divided into three categories: 1, a Northern, or Boreal,, 

 group; 2, a Pan-Tropical group, and, 3, a Southern group (cf. Mayr,. 

 1946, p. 13; Ekman, 1953, pp. 186, 329, has an essentially similar primary 

 division of the invertebrate and fish faunas). 



I am not concerned here with the ultimate origin of particular 

 groups, but of the more immediate relationships of species and genera. 

 Thus, though all petrels might be considered, from the standpoint of 

 ultimate origins, to be ranked as part of a southern fauna, the existing 

 species of some gioups, such as the storm-petrels of the genus Oceano- 

 droma and the Puffinus piiffimis complex, have certainly undergone 

 secondary differentiation in the northern temperate regions, and are 

 considered in this review as northern elements. Similarly the group 

 of races of Puffinus Iherminieri, though they are clearly derived from 

 a southern ancestor (P. assimilis) have undergone a cycle of secondary 

 evolution in the tropical seas of two oceans and may conveniently be 

 considered as members of the pan-tropical fauna. Ekman (1953, 

 p. 200, footnote) was faced with a similar problem in dealing with the 

 marine invertebrates and fishes. 



