292 A HISTORY OF BIRDS 



such descriptions. Happily, however, a few workers have paid 

 some attention to this matter, but such observations are, neces- 

 sarily, extremely limited. And the same is true with regard 

 to the variations of colour and pattern of the plumage. 



Those who fall under this indictment may plead that varia- 

 tion among birds is a negligible quantity ; and it must be 

 admitted that, in so far as most species are concerned, variation 

 appears to be but slight. It is probable, however, that when 

 closer attention is paid to this matter even the most uniform 

 types will be found to vary more than has been supposed. 



In summarising what has been done towards the collection 

 of data on the subject of variation, continuous and discon- 

 tinuous, we may begin with the facts collected by Mr. J. A. 

 Allen who some six-and-thirty years ago published a series of 

 most valuable observations which have been used by every 

 writer on evolutionary problems since they first appeared. 

 They refer to the subject of continuous variation among birds, 

 and are based on certain external characters. After carefully 

 measuring a large series of each of several species he found 

 ■" 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 ''. Each part, he 

 further shows, varies independently of the other parts, so that 

 when the size varies, the proportions of all the parts vary, often to 

 a much greater amount. Thus the wing and the wing and tail 

 vary not only in actual length, but in the relative proportions 

 of the feathers, so that the contours of the outspread wing and 

 tail vary in shape. Similarly, the bill and the legs vary in 

 length, averaging one-sixth of the whole length, and often 

 reaching one-fourth. 



Dr. Alfred Russel Wallace gives, in his Darwinism, an 

 able summary of Mr. Allen's work, and adds thereto a mass of 

 evidence of his own collecting. This study of variation was 

 carried out on a few species of Passerine birds, from twenty to 

 fifty or sixty examples being taken at random from each 

 species for this purpose. Dr. Wallace, in his summary of the 

 work, points out that, in this matter of measurements if large 

 numbers — thousands or millions — were subjected to the same 



