the external similarity between the last-named birds, in believing that the first-mentioned are truly more intimately related the one to the other?
It may be worth while taking a rapid glance at what some of these most important anatomical resemblances and differences happen to be. One of them is the manner in which the feathers are arranged on the skin. Most of us know that, unlike the hair upon a cat or other quadruped, the feathers of a bird are not uniformly distributed over the surface of the body, but grow in linear clusters called tracts, with naked intervals, termed spaces, between them. This may be readily verified by plucking, say a Sparrow, and noticing the thick and opaque light-coloured bands formed by the thickening of the skin surrounding the holes out of which the feathers have been extracted. Between these tracts the skin is seen to be thin and translucent, forming naked spaces through which the colour of the underlying muscles is apparent.
The careful study, some five and forty years ago, by the eminent German ornithologist, C.L. Nitzsch, led him to the conclusion, among others, that these feather tracts are arranged upon a very different plan in the Swallows to what they are in the Swifts, whilst in the Sparrows and their allies they very closely resemble the Swallows. Further he showed that in this feature the Swifts and the Humming Birds are almost identical.
Again, the breast-bone or sternum in birds is much expanded to give origin to the powerful muscles of flight. In both the Swallow and the Sparrow, as in passerine birds generally, its usually oblong figure is modified by the presence of two deep notches, one on each side of the keel, in the posterior margin. But in the Swift there are no such notches to be found, the posterior margin being entire, and in other respects it differs from the same bone in the Passeres, whilst in all it resembles the Humming Birds.
In the Sparrow and the Swallow, again, as in the great majority of the passerine birds, there is at the lower end of the trachea or windpipe, where the bronchi which place it in communication with the lungs arise, an elaborate special mechanism which is known as the muscular organ of voice or lower larynx, by which they have the power—although they do not all employ it—of modulating their note so as to produce a song: this is not found in the Swifts.
