PELAGIC BIRD FAUNAS OF THE INDO-PACIFIC OCEANS 463 



transgressions are demanded for some of the hypothetical species-his- 

 tories in the present paper. 



2. The availabiHty of the following four water gaps: 



(a) The Panama gap between North and South America, en- 

 abling a passage between the Pacific and Atlantic Oceans 

 in the equatorial zone. This has been mapped, from var- 

 ious geological authorities, by Mayr (1946, Fig. 1), who 

 states that it was functional from the lower Eocene to the 

 upper Pliocene. 



(b) The Bering water gap establishing a passage between the 

 same two oceans in the northern cool water zone. This 

 gap would tend to be closed during glacial periods and 

 open in the warm inter-glacial intervals, through eustatic 

 lowering and raising of sea-level. Thus its existence would 

 be associated with all the conditions favouring free move- 

 ment between the seabirds of the North Atlantic and 

 North Pacific. 



(c) The Indo-Malayan water gap between the Indian and 

 Pacific Oceans in the equatorial zone. 



(d) The Mediterranean-Indian Ocean Gap served as a means 

 of communication between the two basins until the late 

 Tertiary period. 



Several periods of major global cooling occurred during the Pleis- 

 tocene Ice Ages, thus providing the opportunity— even if not all suc- 

 ceeded—for repeated transgressions of cool-water species across the 

 equatorial belt. The first to propound this explanation for the so-called 

 "bipolar" distribution of marine fauna was Charles Darwin. Regan 

 (1916, p. 16), though without specifically mentioning the Ice Ages, 

 postulated a past cooling of the tropical waters to explain the present 

 distribution of the pilchards {Sardina). L. S. Berg (1933) fully elabo- 

 rated the thesis in respect to Ice Age coolings and K. H. Voous (1949) 

 invoked it for his theory of the evolution of Fulmarus glacialis from 

 the Antarctic F. glacialoides. Hubb's review (1952, but originally put 

 forward in 1949) re-states and extends the general thesis, which ap- 

 pears essential for a rational explanation of sea-bird relations to the 

 major ocean basins. 



Ekman (1953, p. 244 et seq., in a most useful summary of the "bi- 

 polarity" problem) advances cogent arguments for these processes hav- 

 ing taken place even prior to the period of the Pleistocene Ice Ages, 

 namely during the greater part of the Tertiary. He states (p. 257) : 

 "During the Tertiary Period we cannot assume glacial periods as a 

 cause, but perhaps geographical changes influenced the course of the 



