THE ANCESTRY OF BIRDS. 615 



have keeled breast-bones, which imply their descent from forms adapted 

 to true flight. They are linked to the ostriches, however, and there- 

 fore to the still earlier toothed ancestral types, by the South American 

 tinamous, which are intermediate in various anatomical points (too 

 intricate for a lazy man to go into here and now), between the two 

 classes. Put briefly, one may say that these partridge-like Paraguayan 

 birds are ostriches in the bones of their head, but game-birds in those 

 of the breast and body. This line of descent seems to lead us up di- 

 rectly toward the cocks and hens, the pheasants, and the other scrapers. 

 There are more marks of a primitive organization, however, among 

 the penguins, which are almost wingless swimming birds, belonging 

 nearly to the same class as the ducks and geese ; and we have reason 

 otherwise to consider the penguins a very early form, since fowls re- 

 sembling them in many particulars have been unearthed in the upper 

 greensand. Here the wings are reduced to small rudiments, covered 

 with bristly, scale-like feathers, and so rigid that they can be only 

 moved in the mass like fins by a single joint at the base. They are 

 used, in fact, exactly in the same way as the flappers in seals, to assist 

 the bird in diving. The habitual erect attitude of the penguins 

 strongly recalls that of their reptilian ally, compsognathus. From 

 such an incomplete form as this, the gap is not great to the equally 

 erect auks, the guillemots, the grebes, and other web-footed divers, 

 which have short, pointed wings with true quills, but without any 

 extended power of flight. Some species, indeed, can not fly at all, 

 though the puffins and many other kinds can steer their way through 

 the air with comparative ease. Thence to the cormorants, gulls, and 

 ducks the transitions are slight and easy. We are thus led insensibly 

 from almost wingless erect birds, like the penguins, through winged, 

 but mainly swimming forms like the auks and divers, to creatures 

 with such marvelous powers of flight as the frigate-birds, the petrels, 

 and the albatrosses, which pass almost their whole life upon the wing. 

 It must be remembered, however, that in this line of descent the com- 

 paratively wingless forms must be regarded as somewhat degenerate 

 representatives of flying ancestors ; for the presence of a keeled breast* 

 bone almost conclusively proves hereditary connection with fully- 

 winged progenitors. 



By far the greater number of modern birds belong to the still 

 more strictly aerial orders of the perchers, the peckers, and the birds 

 of prey. In almost all these cases, the power of flight is highly de- 

 veloped, and the bird type reaches its highest ideal point of typical 

 excellence. Among the perchers, this perfection of form is best seen 

 in the swallows, whose ceaseless and graceful curved evolutions every- 

 body has seen with his own eyes ; while among tropical varieties of 

 the same type the birds-of-paradise, the sun-birds, and the orioles are 

 the most conspicuous. Among the peckers, our own swifts closely 

 simulate the swallow type, while their American relatives, the hum- 



