The length of the body is not given by Mr.Allen, but as it forms a convenient standard of comparison, it has been obtained by deducting the length of the tail from the total length of the birds as given by him.

The diagram has been constructed as follows:--The twenty specimens are first arranged in a series according to the body-lengths (which may be considered to give the size of the bird), from the shortest to the longest, and the same number of vertical lines are drawn, numbered from one to twenty.

In this case (and wherever practicable) the body-length is measured from the lower line of the diagram, so that the actual length of the bird is exhibited as well as the actual variations of length.

These can be well estimated by means of the horizontal line drawn at the mean between the two extremes, and it will be seen that one-fifth of the total number of specimens taken on either side exhibits a very large amount of variation, which would of course be very much greater if a hundred or more specimens were compared.

The lengths of the wing, tail, and other parts are then laid down, and the diagram thus exhibits at a glance the comparative variation of these parts in every specimen as well as the actual amount of variation in the twenty specimens; and we are thus enabled to arrive at some important conclusions.
We note, first, that the variations of none of the parts follow the variations of the body, but are sometimes almost in an opposite direction.