THE SWALLOW AND THE SWIFT. 219 
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 con- 
clusion, 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 tae Swifts. 
