y (except
entomology); to the moderate size of the majority of birds; and to the
circumstance that the form and dimensions of the wings, tail, beak, and
feet offer the best generic and specific characters and can all be
easily measured and compared. The most systematic observations on the
individual variation of birds have been made by Mr. J.A. Allen, in his
remarkable memoir: "On the Mammals and Winter Birds of East Florida,
with an examination of certain assumed specific characters in Birds, and
a sketch of the Bird Faunae of Eastern North America," published in the
_Bulletin of the Museum of Comparative Zoology_ at Harvard College,
Cambridge, Massachusetts, in 1871. In this work exact measurements are
given of all the chief external parts of a large number of species of
common American birds, from twenty to sixty or more specimens of each
species being measured, so that we are able to determine with some
precision the nature and extent of the variation that usually occurs.
Mr. Allen says: "The facts of the case show that a variation of from 15
to 20 per cent in general size, and an equal degree of variation in the
relative size of different parts, may be ordinarily expected among
specimens of the same species and sex, taken at the same locality, while
in some cases the variation is even greater than this." He then goes on
to show that each part varies to a considerable extent independently of
the other parts; so that when the size varies, the proportions of all
the parts vary, often to a much greater amount. The wing and tail, for
example, besides varying in length, vary in the proportionate length of
each feather, and this causes their outline to vary considerably in
shape. The bill also varies in length, width, depth, and curvature. The
tarsus varies in length, as does each toe separately and independently;
and all this not to a minute degree requiring very careful measurement
to detect it at all, but to an amount easily seen without any
measurement, as it averages one-sixth of the whole length and often
reaches one-fourth. In twelve species of common perching birds the wing
varied (in from twenty-five to thirty specimens) from 14 to 21 per cent
of the mean length, and the tail from 13.8 to 23.4 per cent. The
variation of the form of the wing can be very easily tested by noting
which feather is longest, which next in length, and so on, the
respective feathers being indicated by the numbers 1, 2, 3, etc.,
commencing