The characters that distinguish birds from mammals on the one hand, and from reptiles on the other, are more apparent than real. Thus flight, the most striking of a bird's gifts, is shared by bats among mammals. Egg-laying is the habit of most reptiles and of three mammals (the Australian duckbill and the echidnas). But incubation by one or both of the parents is peculiar to birds, though the python is said to coil on its eggs.
Birds breathe more rapidly than either mammals or reptiles, and their pneumatieity, or power of inflating numerous air-sacs and even certain bones, is unique.
The temperature of birds ranges from 100° to 112°, while in mammals it reaches 98° to 100°, and in the comparatively cold-blooded reptiles it averages only 40°.
The skull in mammals articulates with the last vertebra (atlas) by two condyles or balls ; in birds and reptiles by only one. In mammals and birds the heart has four chambers ; in reptiles it has but three.
Mammals and reptiles both have teeth, a character possessed by no existing bird; but fossil birds apparently prove that early in the development of the class all birds had teeth.
Thus we might continue the comparison, finding that birds have no universal peculiarities of structure which are not present in some degree in either mammals or reptiles, until we come to their external covering. The reptile is scaled, and so is the fish ; the mammal is haired, and so are some insects ; but birds alone possess feathers. They are worn by every bird—a fit clothing for a body which is a marvelous combination of beauty, lightness, and strength.
There is good evidence for the belief that birds have descended from reptilian ancestors. This evidence consists of the remains of fossil birds, some of which show marked reptilian characters and, as just said, are toothed.
