| Length.
-- -------+-----------+----------+-----------+----------+---------- 2 | 3 | 1 | 4 | 5 | 6 3 | 2 | 4 | 1 | 5 | 6 | / 2 | | | | 3 | { | 1 | 5 | 6 | 7 | \ 4 | | | | 2 \ | | | | | } | 4 | 1 | 5 | 6 | 7 3 / | | | | | 2 \ | | | | | 1 | | | | | | } | 5 | 6 | 7 | 8 | 9 3 | | | | | | 4 / | | | | | -- -------+-----------+----------+-----------+----------+---------- Here we have five very distinct proportionate lengths of the wing feathers, any one of which is often thought sufficient to characterise a distinct species of bird; and though this is rather an extreme case, Mr.
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.

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.