CONVERGENT EVOLUTION 449 



means confined to the sea. The common Black-headed Gull, 

 for example, during the autumn and winter months, contrives 

 to pick up a living by following the plough in search of worms. 

 For at least six months in the year they are to be found in 

 considerable numbers up the Thames, many miles from its 

 mouth, proving a source of infinite pleasure to the jaded toilers 

 of the great city. 



But with the Petrels this is otherwise. They may be said 

 never to leave the sea save for breeding purposes. Many species 

 are crepuscular. As might be expected, in consequence of their 

 strictly marine habits, this group exhibits a series of gradations 

 presenting an increasing specialisation to this habitat. The 

 more generalised fly well, and swim comparatively little. These 

 have relatively short bodies and long legs and wings, such as 

 the Fulmar {Fulmaris glacialis). Storm-petrel (Thallasidroma 

 pelagica) and the Fork-tailed Petrel (Cymochorea leucorrhod) for 

 example. From these we may pass to a long-bodied, short- 

 legged type, such as our Manx Shearwater {Puffinus an- 

 glorum), or the Dusky Shearwater {Puffinus obscurus). In these 

 birds the wings are relatively shorter, and the legs placed further 

 backwards. The former character has followed a lessened use 

 of the power of flight, the latter an increased use of the limbs 

 in swimming ; and these birds pass most of their time on the 

 water, and can dive well, though they appear to resort but 

 rarely to the latter practice. The long-legged species are not 

 known to dive. That this swimming type possessing some 

 powers of diving, and commonly found far from land, should 

 have given rise to a yet more perfect diving form is not sur- 

 prising. This is found in the little Diving Petrels of the 

 Genus Pelecanoides. These birds, of which there are three 

 species, would rather, at first sight, be regarded as some species- 

 of Auk — a highly specialised Plover to be described immediately 

 — than as Petrels. Moseley, in his Voyage of the Challenger^ 

 describes these birds as so numerous in Royal Sound, Ker- 

 guelen's Land, that the water, on two occasions, "was covered' 

 with these birds in flocks extending over acres, which were 

 black with them ". They dive, he says, with extreme rapidity, 

 and when frightened, get up and flutter along close to the 

 water, and drop and dive again. 

 ■ As touching the Auks and Divers. The Auks — by which 

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