ans which
occur in five species taken almost at random from this catalogue. Here,
again, we perceive that the variation is decidedly large, even among a
very small number of specimens; while the facts all show that there is
no ground whatever for the common assumption that natural species
consist of individuals which are nearly all alike, or that the
variations which occur are "infinitesimal" or even "small."

_The proportionate Number of Individuals which present a considerable
amount of Variation._

The notion that variation is a comparatively exceptional phenomenon, and
that in any case considerable variations occur very rarely in proportion
to the number of individuals which do not vary, is so deeply rooted that
it is necessary to show by every possible method of illustration how
completely opposed it is to the facts of nature. I have therefore
prepared some diagrams in which each of the individual birds measured is
represented by a spot, placed at a proportionate distance, right and
left, from the median line accordingly as it varies in excess or defect
of the mean length as regards the particular part compared. As the
object in this set of diagrams is to show the number of individuals
which vary considerably in proportion to those which vary little or not
at all, the scale has been enlarged in order to allow room for placing
the spots without overlapping each other.

In the diagram opposite twenty males of Icterus Baltimore are
registered, so as to exhibit to the eye the proportionate number of
specimens which vary, to a greater or less amount, in the length of the
tail, wing, tarsus, middle toe, hind toe, and bill. It will be noticed
that there is usually no very great accumulation of dots about the
median line which shows the average dimensions, but that a considerable
number are spread at varying distances on each side of it.

In the next diagram (Fig. 10), showing the variation among forty males
of Agelaeeus phoeniceus, this approach to an equable spreading of the
variations is still more apparent; while in Fig. 12, where fifty-eight
specimens of Cardinalis virginianus are registered, we see a remarkable
spreading out of the spots, showing in some of the characters a tendency
to segregation into two or more groups of individuals, each varying
considerably from the mean.

[Illustration: FIG. 9]

[Illustration: FIG. 10.]

[Illustration: FIG. 11.]

In order fully to appreciate the teaching of these di