The Steganopodes are an order which is particularly well represented in the South Pacific and Indian Oceans. The pelicans, gannets, cormorants, darter, tropic– and frigate-birds number fifty-four species in all. Thirty-one breed in the South Pacific. Twenty-eight breed in the Indian Ocean. The North Pacific has twenty-three, the South Atlantic twenty, the North Atlantic sixteen, the Mediterranean six, the Antarctic three, and the Arctic two. The present distribution suggests that the order radiated from what is now the East Indian region—from south-east Asia or Australasia.
In the order Laro-Limicolae the family Chionididae, two curious pigeon-like sheathbills, Chionis, are found in Antarctica; and one also breeds in the South Atlantic and South Pacific.
In the family Laridae the gulls (subfamily Larinae) number forty-two. In the North Pacific sixteen of these breed, in the North Atlantic fourteen, in the Arctic eleven, in the South Pacific nine, in the Indian Ocean six, in the South Atlantic five, in the Mediterranean five, in the Antarctic two. Besides these two breed inland only in North America, one inland only in South America, and three inland only in the Palearctic Region. This appears to be the only group of sea-birds whose evolutionary radiation may have taken place from the north; the Arctic and neighbouring parts of the North Pacific and Atlantic appears to be the origin of the gulls. The terns (subfamily Sterninae) number thirty-nine, of which twenty-three breed in the North and twenty-two in the South Pacific, nineteen in the Indian Ocean, nineteen in the North Atlantic, fifteen in the South Atlantic, ten in the Mediterranean, two in the Antarctic, two in the Arctic and one inland only in South America. The radiation of terns appears to be pretty general over the world’s seas, and they may have originated in the tropics, perhaps in the Indian Region. The skuas (subfamily Stercorariinae) have only four species, one of which (Catharacta skua, the great skua) has its breeding-headquarters in the Antarctic; it also breeds in the South Pacific, South and North Atlantic. The other skuas have an arctic breeding-distribution which extends into the North Pacific and North Atlantic. The three skimmers Rynchops belong to a separate family, Rynchopidae; North Atlantic, South Atlantic and South Pacific each have two; the Indian Ocean has one. Some workers regard them as all of one species.
The family Alcidae (the auks) take the place in the north of the penguins of the south. Undoubtedly their origin has been in or not far from the Bering Sea. Of the twenty-two species, sixteen belong to the northern part of the North Pacific, twelve to the Arctic Ocean north of the Circle, and six to the northern part of the North Atlantic.
This concludes the list of sea-birds belonging to groups of super-family or higher status whose evolution has been marine. There are several further (secondarily marine) groups which contain sea-birds, or part-time sea-birds; thus all four members of the order Gaviae the divers, breed in the Arctic, and North Atlantic and Pacific regions, and winter at sea on the coasts of the oceans. Many of the twenty species of grebes, order Podicipedes, are marine outside the breeding-season, and six of them visit the coasts of the North Atlantic at that time. Among the geese and ducks many (see Appendix, see here) are partly marine, and some (e.g. eiders and scoters) are largely marine in the breeding—as well as in the off-season: two eiders and three scoters breed in the North Atlantic-Arctic. Among the waders (Charadriidae) the subfamily Phalaropinae contains only three members, all of which breed in the Arctic, North Atlantic and North Pacific, and two of which winter in the open sea.
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 (16½ per cent.); Arctic 31 (11½ per cent.); Mediterranean 24 (9 per cent.); and purely inland only 7 (2½ 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 Wegener’s theory of the origin of the Atlantic; but whether the Wegener 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 differentiation 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 theoretical work on evolution has also been done. But it needed the persuasions of Mayr and Julian 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 type; and this linear distribution makes it easy to apply Huxley’s concept that the characteristics of animals tend to grade from one part of their range to another in an orderly way. Some of these gradations had been recognised long before Huxley thought of the word “cline” because they are adaptations to the environment. For instance Bergmann’s Rule states that from the warmer parts of an animal’s distribution-area to the colder parts there tends to be an increase in its size. Thus the puffins, black guillemots and eider-ducks of the Arctic are considerably bigger than those of Britain. The main adaptive reason for this is that larger animals have less surface in proportion to their weight, and consequently heat is not lost from them (if warm-blooded) so rapidly as it is from small animals. Another rule, Allen’s Rule, states that warm-blooded animals of cold climates tend to have their heat-radiating surfaces decreased by a reduction in size of their extremities and limbs such as ears, tails, necks, legs and noses. There is also a general tendency (Gloger’s Rule) for animals to become darker as humidity increases.
If we examine those sea-birds which are widely distributed, we find clines in various characteristics, notably in size, i.e. total size, and also size of limbs and extremities, beak-length, wing-length, etc., and in colour. There are also clines in shape; for instance the fulmars of the north-east Atlantic have very thick bills, those of Baffin Island rather more slender bills, those of the North Pacific more slender bills still, and those of the Antarctic very slender bills indeed. No sea-bird is arranged quite evenly in its geographical distribution. Just as the distribution in space is never even, so are the gradations in character never even. From one part of the geographical distribution of a species to the other, change often occurs more as a series of steps rather than as continuous ramp.
Most working ornithologists today will agree that there are more subspecific names about than a true understanding of bird evolution requires. It is the species which has reality and significance. In this book we have tried to be sparing in the use of subspecies, and have rejected some that appear in many current text-books. Nevertheless, a study of the geographical races of the species of the North Atlantic