The Apteryx also, of New Zealand, is quite analogous. The old bird has a nearly continuous covering of feathers, but the unhatched young show perfectly distinct patches of feathers with bare spaces between. We shall show on another page that there are other arguments which appear to prove that all these flightless birds have been gradually derived in the course of time from birds that could fly perfectly well. They are an instance, so far, of what is termed degeneration. The examination of any bird will show that it has several kinds of feathers. They are all constructed upon the same plan, but some are larger than others, and the smallest are soft instead of firm to the touch. [Illustration: FIG. 1.--SKELETON OF WING OF ARCHÆOPTERYX WITH REMIGES ATTACHED. (Restoration after Pycraft, ‘Natural Science,’ vol. v.) I, II, III, digits.] The biggest feathers of all are a set which fringe the wing (see fig. 1) and another set at the end of the tail. These are called respectively the ‘Remiges’ and ‘Rectrices,’ or the ‘rowing’ feathers and the ‘steering’ feathers. Their principal use, as may be imagined, is in flight. The remaining feathers are also to some extent used in flight, but their main use appears to be to keep the body warm. An eider-down quilt, as everybody knows, is the warmest kind of coverlet; the reason being that the feathers are very bad conductors of heat, and do not, therefore, allow the heat of the body to escape. Birds are the hottest of all animals, which is in part due to their covering of feathers. To understand the structure of a typical feather is perhaps a little difficult; but possibly the accompanying figures (figs. 1, 2, 3, 4) will render the explanation easier to follow. The feather consists of a stem which is technically called the rhachis, the word simply signifying stem. From each side of this a row of parallel rodlets arise which are called barbs. These in their turn give rise to another set of processes which are the barbules. This, however, is not all; the barbules are firmly locked together by other processes, so that the entire feather is quite firm, and can be used as a kind of oar with which to row through the air. It does not give when the wings are flapped. The barbules are of two sorts, those nearest to the root of the barb being different from those which are nearest to its tip. The former, as is shown in fig. 2, are shaped something like a knife-blade; they are thickened above and bent in the middle; they gradually taper away to a fine point. Just in the middle, where the bend is, are two or three small teeth (2, fig. 2) on the upper margin. By means of these teeth-like processes the successive barbules are attached to one another. At the end of each barb, as already mentioned, the barbules are of a different structure. A few of them are illustrated in fig. 4. The end is frayed out into a number of delicate spines, of which those farthest from the actual tip are hooked, while those at the tip are