EVOLUTION 37 



If we ignore these secondary sea-birds, and consider the 267 

 species of the primary marine groups, we find that the hierarchy is 

 this: South Pacific 128 (51 per cent.) ; North Pacific 107 (40 per cent.) ; 

 North Atlantic 74 (28 per cent.); South Atlantic 73 (27 per cent.); 

 Indian Ocean 73 (27 per cent.); Antarctic 44 (16 J per cent.); Arctic 

 31 (11 J per cent.); Mediterranean 24 (9 per cent.); and purely inland 

 only 7 (2 J per cent.). 



It can be seen that the North Atlantic, with its seventy-four 

 species, is much lower than either half of the Pacific than would appear 

 warranted by its area. There is not the faintest hint, from the radiation 

 of any of the sea-bird groups, that either North or South Atlantic has 

 been the arena of any great evolutionary changes. The Atlantic has 

 been colonised from without; by penguins from the Antarctic; by 

 petrels from the South Pacific; by pelecaniform birds and terns 

 probably from the Indian Ocean; by gulls and auks from the Arctic. 

 The North Atlantic and the immediately neighbouring parts of the 

 Arctic have but two present sea-bird genera and only thirteen species 

 of their own. We need not be surprised at this indication that the 

 Atlantic's bird fauna is derived from that of other oceans if we accept 

 \Vegener's theory of the origin of the Atlantic; but whether the Weg- 

 ener theory is true or not it is quite clear that the North Atlantic 

 has not been the home in which any important group of sea-birds has 

 evolved. This is not to say that there has been no sea-bird evolution 

 in the North Atlantic; but it has not usually gone beyond the differ- 

 entiation of species. Of this it has, indeed, much to show. Some of 

 the classic examples which E. Mayr (1942) has discussed are North 

 Atlantic species. Mayr's thesis is that one species can only become 

 two after it has been differentiated geographically. He opposes the 

 notion which has found favour in some quarters that speciation may 

 occur by ecological differentiation or by the differentiation of behaviour. 



So far the available evidence appears to uphold Mayr's view — 

 at all events, for birds. During the present century much systematic 

 work in the description and measurement of birds has been conducted 

 in American and European museums, and much practical and theoreti- 

 cal work on evolution has also been done. But it needed the persuasions 

 of Mayr and JuHan Huxley (1942), amongst a few others, to collate 

 the work of the systematists and the evolutionary zoologists. Sea-birds 

 lend themselves to evolutionary study because they are so largely 

 confined to coasts for breeding purposes. This makes their distribution 

 often linear rather than of the ordinarily spatial two-dimensional 



