with the outer one. As an example of the irregular variation
constantly met with, the following occurred among twenty-five specimens
of Dendroeca coronata. Numbers bracketed imply that the corresponding
feathers were of equal length.[20]

RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
DENDROECA CORONATA.
---------+-----------+----------+-----------+----------+----------
Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
| Length. | Length. | Length. | Length. | 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