Allen assures us that "the comparison, extended in the table to only a
few species, has been carried to scores of others with similar results."
Along with this variation in size and proportions there occurs a large
amount of variation in colour and markings. "The difference in intensity
of colour between the extremes of a series of fifty or one hundred
specimens of any species, collected at a single locality, and nearly at
the same season of the year, is often as great as occurs between truly
distinct species." But there is also a great amount of individual
variability in the markings of the same species. Birds having the
plumage varied with streaks and spots differ exceedingly in different
individuals of the same species in respect to the size, 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.
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