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 glance.
[Illustration: FIG. 3.--Variation of Wings and Tail.]
There is here no question of "minute" or "infinitesimal" variation,
which many people suppose to be the only kind of variation that exists.
It cannot even be called small; yet from all the evidence we now possess
it seems to be the amount which characterises most of the common species
of birds.
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