DIVISIONS OF BIRDS. 267 



tre of gravity of the body ; this position enabling them to act 

 admirably as swimming-paddles, at the same time that it ren- 

 ders the gait upon dry land comparatively awkward and shuf- 

 fling. The toes in all the Natatores are webbed to a greater 

 or less extent, or, in other words, are united by a membrane 

 (Fig. 131). In many the web or membrane between the toes 

 is stretched completely from toe to toe, but in others the mem- 

 brane is divided between the toes, so that the feet are only 

 imperfectly webbed. As their aquatic mode of life exposes 

 them to great reductions of temperature, the body in the Na- 

 tatorial birds is closely covered with feathers, with a thick 

 covering of down next the skin. They are further protected 

 against becoming wet while in the water by the great develop- 

 ment of the oil-gland at the tail, by means of which the dense 

 plumage is kept constantly oiled. As a rule, the Natatorial 

 birds are polygamous, each male having several females ; and 

 the young are hatched in a condition not requiring assistance 

 from their parents, being able to swim about and procure food 

 for themselves the instant they are liberated from the egg. 



Among the more important families of the Natatores may 

 be enumerated the Penguins (Spheniscidce) , the Auks (Al- 

 cidce), the Gulls and Terns (Laridce), the Petrels (Procella- 

 ridoe}, the Pelicans (Pelicanus), the Cormorants (Phalacro- 

 corax), the Gannets (Sula), the Ducks (Anatidce), the Geese 

 (AnserincB\ and the Swans ( CygnidcB). 



The Penguins and Auks, with their allies the Divers, Guil- 

 lemots, and Grebes, have rudimentary, or at any rate small, 

 wings, and are all more at home in the water than upon land. 

 The Gulls, Terns, and Petrels, on the contrary, are all birds of 

 powerful flight, and some of them, such as the Albatross, are 

 habitually found hundreds of miles from the nearest land. 

 The Pelicans, with their allies the Cormorants, Frigate-birds, 

 and Darters, are excellent flyers, and aleo not uncommonly 

 perch on trees, which few Natatorial birds do. They are dis- 

 tinguished by having the hinder toe directed inward, and 

 united to the innermost of the front toes by a continuous 

 membrane. The Gannets, Ducks, Geese, and Swans, have the 

 bill very much flattened and covered by a soft skin. The 

 edges of the bill are also furnished with a series of transverse 

 plates, which form a kind of fringe or " strainer," by means 

 of which these birds sift the mud in which they habitually 

 seek their food. 



OEDEB II. GKALLATOEES (Lat. grattce, stilts). The Wading 

 Birds for the most part frequent moist situations, such as 



