50 
DARWINISM 
CHAV. 
stance that the form and dimensions of the wings, tail, beak, 
and feet offer the best generic and specific characters and can 
all be easily measured and compared. The most systematic 
observations on the individual variation of birds have been 
made by Mr. J. A. Allen, in his remarkable memoir: “On the 
Mammals and Winter Birds of East Florida, with an examina¬ 
tion of certain assumed specific characters in Birds, and a 
sketch of the Bird Faunae of Eastern North America,” 
published in the Bulletin of the Museum of Comparative 
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 part varies 
to a considerable extent independently of the other parts; so 
that when the size varies, the proportions of all the parts 
vary, often to a much greater amount. The wing and tail, 
for example, besides varying in length, vary in the pro¬ 
portionate 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, as 
does each toe separately and independently ; and all 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 variation of the form of 
the wing can be very easily tested by noting which feather is 
longest, which next in length, and so on, the respective 
feathers being indicated by the numbers 1, 2, 3, etc., com- 
