. Bird-life; a guide to the study of our common birds . las) by two condyles or balls; in birds and reptilesby only one. In mammals and birds the heart has fourchambers; in reptiles it has but three. Mammals and reptiles both have teeth, a characterpossessed by no existing bird; but fossil birds appar-ently prove that early in the development of the classall birds had teeth. Thus we might continue the comparison, finding thatbirds have no universal peculiarities of structure whichare not present in some degree in either mammals orreptiles, until we come to their external covering. Thereptile i
. Bird-life; a guide to the study of our common birds . las) by two condyles or balls; in birds and reptilesby only one. In mammals and birds the heart has fourchambers; in reptiles it has but three. Mammals and reptiles both have teeth, a characterpossessed by no existing bird; but fossil birds appar-ently prove that early in the development of the classall birds had teeth. Thus we might continue the comparison, finding thatbirds have no universal peculiarities of structure whichare not present in some degree in either mammals orreptiles, until we come to their external covering. Thereptile is scaled, and so is the fish ; the mammal is haired,and so are some insects; but birds alone possess are worn by every bird—a fit clothing for a bodywhich is a marvelous combination of beauty, lightness,and strength. There is good evidence for the belief that birds havedescended from reptilian ancestors. This evidence con-sists of the remains of fossil birds, some of which showmarked reptilian characters and, as just said, are Plate II. Page 84. PIED-BILLED GREBE. Length, 13-50 inches. Summer plumage, upper parts blackish brown; throat and spot on bill blacli; fore neck brownish, rest of under parts grayish white. Winter plumage, similar, but without black on throat or bill. ANCESTORS OF BIRDS. 3 It is unnecessary to discuss here the relationships of thebirdhke reptiles, but, as the most convincing argumentin support of the theory of the reptilian descent of birds,I present a restoration of the Archaeopteryx, the earliestknown progenitor of the class Aves. This restoration is
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