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Show 56 DARWINISM 5 10 15 20 25 5 10 15 20 2o F10. 5.-Agelreus phreniccus. OHAP. 30 35 40 ao 35 40 40 Males. III Y .ARIABILITY OF SPECIES IN A. STATE OF NATURE 57 sitlorcd to give tho size of tho bird), from tho shortest to tho longe. t, and the same number of vertical lines arc dmwn, numbered. from one to twenty. In this case (and wherever practicable) tho body-length is measured from the lower line of tho <.liagram, so that tho actual length of the bird is exhibited as well as tho actual variations of length. These can be well estimated by means of the horizontal line drawn at tho moan between tho two extremes, and it will be seen that one-fifth of the total number of specimens taken on either side exhibits a very largo amount of variation, which would of course be very much greater if a hundred or more specimens wore compared. Tho lengths of the wing, tail, an l other parts arc then laid down, and tho diagram thus exhibits at a glance the comparative variation of the. c parts in every specimen as well as tho actual amount of variation in the twenty specimens; and. we arc thus enabled to arrive at some important conclusions. vV e note, first, that the variations of none of tho parts follow . the variations of tho body, but arc sometimes almost in an opposite direction. Thus the longest wing corresponds to a rather small body, the longest tail to a medium body, while tho longest log and toes belong to only a moderately largo body. Again, even related parts do not constantly vary toO'ethcr but present many instances of independent variatio11, as shown by the want of parallelism h1 their respective variation-lines. In No. 5 (sec Fig. 4) the ·wing is very long, the tail moderately so ; while in No. 6 the wing is much hortcr w bile the tail is considerably longer. Tho tarsus presents comparatively little variation; and although the three toes may be said to vary in general together, there arc many divergencies; thus, in passing from No. 9 to No. 10, the outer toe becomes longer, w bile tho hind toe becomes considerably shorter; while in Nos. 3 and 4 the middle toe varies in an opposite way to the outer and the hind toes. In the next diagram (Fig. 5) we have the variations in forty males of the Hod-winged Blackbird (Agelrous phamiceus), and here we see the same general fe:.tturcs. One-fifth of the whole number of specimens off'er a large amount of variation either below or above the mean ; while the wings, tail, and head vary quite independently of the body. The wing and tail too, |