, shape, and
number of these marks, and in the general aspect of the plumage
resulting from such variations. "In the common song sparrow (Melospiza
melodia), the fox-coloured sparrow (Passerella iliaca), the swamp
sparrow (Melospiza palustris), the black and white creeper (Mniotilta
varia), the water-wagtail (Seiurus novaeboracencis), in Turdus
fuscescens and its allies, the difference in the size of the streaks is
often very considerable. In the song sparrow they vary to such an extent
that in some cases they are reduced to narrow lines; in others so
enlarged as to cover the greater part of the breast and sides of the
body, sometimes uniting on the middle of the breast into a nearly
continuous patch."

Mr. Allen then goes on to particularise several species in which such
variations occur, giving cases in which two specimens taken at the same
place on the same day exhibited the two extremes of coloration. Another
set of variations is thus described: "The white markings so common on
the wings and tails of birds, as the bars formed by the white tips of
the greater wing-coverts, the white patch occasionally present at the
base of the primary quills, or the white band crossing them, and the
white patch near the end of the outer tail-feathers are also extremely
liable to variation in respect to their extent and the number of
feathers to which, in the same species, these markings extend." It is to
be especially noted that all these varieties are distinct from those
which depend on season, on age, or on sex, and that they are such as
have in many other species been considered to be of specific value.

These variations of colour could not be presented to the eye without a
series of carefully engraved plates, but in order to bring Mr. Allen's
_measurements_, illustrating variations of size and proportion, more
clearly before the reader, I have prepared a series of diagrams
illustrating the more important facts and their bearings on the
Darwinian theory.

The first of these is intended, mainly, to show the actual amount of the
variation, as it gives the true length of the wing and tail in the
extreme cases among thirty specimens of each of three species. The
shaded portion shows the minimum length, the unshaded portion the
additional length in the maximum. The point to be specially noted here
is, that in each of these common species there is about the same amount
of variation, and that it is so great as to be obvious at a glan