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. Thus the longest wing corresponds to a rather small body, the
longest tail to a medium body, while the longest leg and toes belong to
only a moderately large body.
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