riation, yet in
no less than nine cases vary in opposite directions as compared with the
preceding species.

The next diagram (Fig. 6), showing the variations of thirty-one males of
the Cardinal bird (Cardinalis virginianus), exhibits these features much
more strongly. The amount of variation in proportion to the size of the
bird is very much greater; while the variations of the wing and tail not
only have no correspondence with that of the body but very little with
each other. In no less than twelve or thirteen instances they vary in
opposite directions, while even where they correspond in direction the
amount of the variation is often very disproportionate.

As the proportions of the tarsi and toes of birds have great influence
on their mode of life and habits and are often used as specific or even
generic characters, I have prepared a diagram (Fig. 7) to show the
variation in these parts only, among twenty specimens of each of four
species of birds, four or five of the most variable alone being given.
The extreme divergence of each of the lines in a vertical direction
shows the actual amount of variation; and if we consider the small
length of the toes of these small birds, averaging about three-quarters
of an inch, we shall see that the variation is really very large; while
the diverging curves and angles show that each part varies, to a great
extent, independently. It is evident that if we compared some thousands
of individuals instead of only twenty, we should have an amount of
independent variation occurring each year which would enable almost any
modification of these important organs to be rapidly effected.

[Illustration: FIG. 7.--Variation of Tarsus and Toes.]

[Illustration: FIG. 8.--Variation of Birds in Leyden Museum.]

In order to meet the objection that the large amount of variability here
shown depends chiefly on the observations of one person and on the birds
of a single country, I have examined Professor Schlegel's Catalogue of
the Birds in the Leyden Museum, in which he usually gives the range of
variation of the specimens in the museum (which are commonly less than a
dozen and rarely over twenty) as regards some of their more important
dimensions. These fully support the statement of Mr. Allen, since they
show an equal amount of variability when the numbers compared are
sufficient, which, however, is not often the case. The accompanying
diagram exhibits the actual differences of size in five org