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Show 50 DARWINISM Cit' AI'. stance that the form and dimensions of the wings, tail, hea.k, and feet offer the best generic and specific characters and C<trl all be easily measured and compared. The most systematic observations on the individun1 variation of birds have been made by Mr. J. A .. Allen, in his rcmarlmble memoir: "On the Mammals and Winter Birds of East Florida, with an cxn.mina,tion of certain assumed specific characters in Birds, and a sketch of the Bird Faunre of Eastern North America," published in the B'ttlletin of the Museum of Cornpamtive Zoology at Harvard College, Cambridge, Massachusetts, in 1871. In this work exact measurements are given of all the chief external parts of a large number of species of common American birds, from twenty to sixty or more specimens of each species being measured, so that we are able to determine with some precision the nature and extent of the variation that usually occurs. Mr. Allen says : " The facts of the case show that a variation of from 15 to 20 per cent in general size, and an equal degree of variation in the relative size of different parts, may be ordinarily expected among specimens of the same species and sex, taken at the same locality, while in some cases the variation is even greater than this." He then goes on to show that each paxt varies to a considerable extent independently of the other p:ut. ; so that when the size varies, the proportions of all the pn.rts vary, often to a much greater amount. The wing and tail, for example, besides varying in length, vary in the proportionate length of each feather, and this causes their outline to vary considerably in shape. The bill also varies in length, width, depth, and curvature. The tarsus varies in length, a. docs each toe separately and independently; and n.ll this not to a minute degree requiring very careful measurement to detect it at all, but to an amount easily seen without any measurement, as it averages one-sixth of the whole length and often reaches one-fourth. In twelve species of common perching birds the wing varied (in from twenty-five to thirty specimens) from 14 to 21 per cent of the mean length, and the tail from 13 ·8 to 23 ·4 per cent. The vnriation of the form of the wing can be very easily tested by 11oting which fcuther is longest, which next in length, and so on, the re:pective feathers being indicated by the numbers 1, 2, 3, etc., com- Ill VARIABILITY OF SPECIES IN A STATE OF NATURE 51 mc~ci~g with the outer one. As an example of the irregular vanat10n constant~y met with, the following occurred among twenty -five speCimens of Dendrreca coronata. Numbers bracketed imply that the corresponding feathers were of equal length. I RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF DENDltJECA CORONATA. Longest. Second in I Third in F ourth in Fiflh in Sixth in J ... cngLh. Length. L<•nglh. LcngLh. Length. 2 3 1 4 5 6 3 2 4 1 5 6 3 l 2 4 1 5 6 7 2 } 4 1 3 5 6 7 2 31 ~ 5 6 7 ' 8 9 4 Here we have five very distinct proportionate lenaths of the wing feathers, any one of which is often thought s~fficient to characterise a distinct species of bird ; and though thi: i rather an extreme case, Mr. Allen assures us that "the compari~ on, extended in the table to only a few species, bas been earned to scores of others with similar results." Along with this variation in size and proportions there occurs a large amount of variation in colour and markings. "Tho difference in intensity of colour between the extremes of a series of fifty or one hundred specimens of any species, collected ~t a single locality, and nearly at the same season of the year, Is often as great as occurs between truly distinct species." But there is also a great amount of individual variability in the ma1:kings. of the same species. Birds having the plumage ;ar~e~ w1th streaks and spots differ exceedingly in different md1v1duals of the same species in respect to the size, shape, and number of these marks, and in the general aspect of the plumage resulting from such variations. "In the common 1 See Winter Birds of Fl01'ida, p. 206, Table F. |
