A feather consists of a quill, a shaft, barbs, and barbules: moreover, there may be a kind of accessory part, often in the shape of a downy tuft, close to the junction of the shaft and the quill. The shaft (scapus) or axis of every perfect feather (penna) is divided into the quill (calamus), the hollow cylinder (d), which is partly embedded in a sac of the skin, and the true shaft (rachis, a), which bears on each side the lateral processes called barbs (rami or radii). The rachis and the barbs together are known as the vane (vexillum), and, in fact, form what is commonly known as the “feather” in contradistinction to the “quill.” The barbs (c, c, c, c) are narrow plates, or laminæ, “tapering to points at their free ends, and attached by their bases on each side of the rachis. The edges of these barbs are directed upwards and downwards, when the vexillum of the feather is horizontal The interstices between the barbs are filled up by the barbules, pointed processes, which stand in the same relation to the barbs as the barbs do to the rachis. The barbules themselves may be laterally serrated and terminated by little hooks, which interlock with the hooks of the opposed barbules. In very many birds each quill bears two vexilla; the second, called the aftershaft (b) (hyporachis), being attached on the under side of the first,”[133] close to the junction of the shaft with the quills.
In all the feathers of the Ratitæ, and in the case of all but the contour feathers in other birds, there are no barbules to the barbs. The pennæ are ordinarily arranged in definite patches, or areas on the bird, and the shape and size of these, and their relation to one another, differ in many birds.
The aftershaft (b) is ordinarily a smaller vexillum, which is attached to the under side of the larger one at about the point where the rounded quill passes into the stem.
It is not necessary to notice these important characteristic structures more fully now, as they will have to be considered in explaining the distinctions between the great groups of birds, and we pass on to notice that the same kinds of birds are not found everywhere, but that they have, as groups, a remarkable geographical distribution.
In the following pages the distribution of birds is often alluded to, although it will naturally be impossible to discuss, within these limits, all the various phases of the study which the geographical distribution of the feathered tribes opens up to us. At the same time sufficient evidence will be given to show that birds are not scattered without order over the earth, but are more or less restricted to certain spots.
PARTS OF A FEATHER. (After Nitzsch.)
The six natural history or distributional provinces into which the world is ordinarily divided by modern naturalists were determined, first of all, from the study of the birds; and in fixing the boundaries of each division the wading birds and many swimming birds must be left out of the question, as they are creatures of such very extensive flight, and wander almost from pole to pole. A natural region, therefore, can be marked only by its resident forms of bird life, or at the most by the birds which breed within its limits; and the six regions alluded to provide us with many excellent reasons for believing that they possess well-defined physical boundaries. No Capercailzie, for instance, was ever found out of the Palæarctic[134] region, which comprises Europe and the greater part of Asia above the line of the Himalayas and the Yangtze-kiang River in China. This region is also characterised by a large number of Buntings, Warblers, Grouse, &c. In the Nearctic[135] region there is a certain similarity to the European and Siberian Avifauna, Grouse, Ptarmigan, Waxwings, Magpies, Ravens, &c., being commonly found throughout the two regions. North America possesses, however, several forms peculiar to itself, though it is by no means so rich in species as is the Neotropical[136] region, which commences south of a line drawn through Northern Mexico, and includes the whole of Central and Southern America. Within this large area are contained whole families of birds, such as Toucans, Mot-mots, the vast majority of the Humming-birds, Trogons, besides innumerable genera of Tanagers and other forms, so that this region is by far the richest in the world as regards bird life. The Ethiopian region embraces all Africa below the Sahara Desert and Madagascar: Plaintain-eaters, &c., are characteristic of this region. The Indian region skirts the Palæarctic, and includes the remainder of Asia below the Himalayas and the Yangtze-kiang; the Malayan Peninsula, the Sunda Islands, and the Philippines, belong to this region, which contains all the finest Pheasants in the world, the Impeyan Pheasant from the Himalayas, the Tragopans, and the Lobed Pheasant of Borneo being most beautiful creatures. Lastly, between the islands of Bali and Lombok passes a deep sea boundary called “Wallace’s line,” which divides the Australian region from the Indian, and although these islands lie so close together, the great depth of the channel between them seems to mark them out as frontier lines of two ancient continents. Certain it is that the birds and animals on each side of Wallace’s line differ remarkably; and the Australian region, which includes all the Moluccas, New Guinea, and Oceania, in addition to the Australian continent and New Zealand, presents us with forms not found elsewhere, such as Birds of Paradise, Cassowaries, Lyre-birds, and a large variety of peculiar types. Many smaller divisions of the globe are now recognised, but the above are the main ones, which may occasionally be referred to in these pages.[137]
Many birds migrate, and the student of migration alone would find sufficient material there for the work of a lifetime; and it seems almost impossible to account for the instinct or other causes which bring birds regularly year by year to breed in the same haunts, and which drive them away at the same change of season. Why is it, for instance, that species of similar habits and form, and both visiting Europe in equal abundance, should occupy such different winter quarters? Yet the common Red-backed Shrike, or Butcher-bird (Lanius collyrio), when he is said to leave Europe, passes by the Nile Valley along the east coast of Africa down to the Cape, where he brings up a second brood of nestlings; while the Wood-Chat Shrike (Lanius auriculatus), a bird of about the same size and of precisely similar habits, proceeds down the Nile Valley and invades Abyssinia in the winter, and also occupies Senegambia, where a Red-backed Shrike has never been found yet by a naturalist. Nothing whatever is known by which route the bird gets to the Gambia: whether he follows the same one as his red-backed relation as far as Abyssinia, and then skirts the southern edge of the Sahara, or whether he reaches north-western Africa by a direct flight across the Great Desert. Many other such problems in the economy of our most familiar species are still awaiting further scientific research.