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 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.
It may be said, however, that these are the extreme variations, and only occur in one or two individuals, while the great majority exhibit little or no difference.

Other diagrams will show that this is not the case; but even if it were so, it would be no objection at all, because these are the extremes among thirty specimens only.