50 DARWINISM chap. 



stance that the form and dimensions of the ■n^ings, 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 ComjMrative 

 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 

 Avith 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 joer 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 consideraljle 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 A\athout any 

 measurement, as it averages one-sixth of the whole length and 

 often reaches one -fourth. In twelve species of common 

 perching birds the Aving varied (in from twenty-five to thirty 

 specimens) from 1 4 to 2 1 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- 



