[19]
_Variation among Birds_.
Coming now to the class of Birds, we find much more copious evidence of
variation. This is due partly to the fact that Ornithology has perhaps a
larger body of devotees than any other branch of natural history (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.
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