Page:Popular Science Monthly Volume 15.djvu/383

 itself. But perhaps it is premature to build with any confidence upon such dubious ground; and we may consequently accept the earliest birds on their own responsibility, without inquiring too curiously into their antecedents, or compelling them to produce a genealogical table of their ancestry.

The essential characteristic of a bird consists in the fact that it is a flying animal; and feathers are the kind of skin-covering best adapted to its special manner of life. In their nature and mode of development, feathers closely agree with the hair of mammals; but the differences between them are all of a sort which fit the bird for its aërial existence. We see this fact very clearly if we look at the instance of those birds which do not fly. Running species, such as the ostriches, have downy plumes, in which many of the essential characters of the feather are greatly obscured. In the emu, whose habits are more strictly cursorial, the plumage almost resembles hair. In the cassowary the likeness becomes yet more striking, while the wingless apteryx of New Zealand has not even the few bare quills which stand for wing-feathers in the former bird. So, too, among those sedentary marine birds, the penguins, where the wings have been converted into a sort of fins for diving, the feathers undergo a parallel change into scales. There is reason, indeed, to suspect, as Mr. Lowne has pointed out, that these marine species retain in many ways the primitive characters of the class; and we may perhaps regard them rather as birds in whom the pinions and plumage have never fully developed than as birds in whom they have assumed a new form.

On the other hand, the truest feathers—that is to say, those which exhibit the essential features of a feather in the most marked manner—are specially connected with the act of flight. The general surface of the body is covered with soft down, among which sprout the delicate plumes that form the common covering for warmth and protection; but only on the wings and tail do those long and stiff quills appear which, after all, are the feathers par excellence, the models and prototypes of all the rest. Now, it is quite obvious to every one that the wings are the organs of flight, and that the quills are the part by means of which the powerful muscles of the bird are brought to bear upon the sustaining atmosphere. As for the tail, its functions resemble those of a rudder, in directing the course of flight to right or left. The difference between these true flying feathers and the mere clothing of the back and breast is so striking that naturalists have given them separate technical names, as quills and plumes respectively.

From such facts, and others like them, I think we may arrive at an important conclusion—that feathers have been developed and selected through the habit of flight. Probably our monstrous friend the pterodactyl had only a membranous wing or bit of skin, extending from the elongated outer finger of his forearm to the leg. Such a parachute we still see among the so-called flying-squirrels and lemurs; while in the