BIRDS AND EVOLUTION 371 



§ 4. Illustrations of Variations 



Fluctuations or continuous small variations may be 

 illustrated in regard to the following characters (Allen, 1871, 

 and Wallace, 1889) : the total full-grown size, the proportions 

 of various parts, the relative lengths of feathers, the strength 

 of particular muscles, the shape of the bill, the colour of 

 the plumage. Allen measured 20-60 specimens of a large 

 number of common American birds and concluded : " The 

 facts of the case show that a variation of from 1 3 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." So there is abundant raw material on 

 which natural selection might operate. Reference has 

 already been made to the extraordinary abundance of slight 

 differences in the coloration of guillemot's eggs, which 

 must correspond to slight variations in the constitution or 

 metabolism of the mother-bird. Lotsy speaks (1916, p. 92) 

 of the collection of 200 specimens of buzzard {Buteo huted) 

 in the Leiden Museum, " hardly two of which are alike." 

 He interprets the diversity as due to the fact that " no 

 selection has been at work, because this bird of prey is so 

 strong that it has practically no enemies in the regions in 

 which it occurs." A famous instance of diversity is that of 

 the ruffs ; the individuality of the male birds is extra- 

 ordinarily pronounced, but all the females or reeves seem 

 to be like one another. 



Mutations or brusque variations may be illustrated by 

 the sudden appearance of a white blackbird, or of a fowl with 

 extra toes. Bateson (1894, p. 55) gives a good example in 

 the Moorhen which occasionally appears as a " hairy variety." 

 The feathers are destitute of barbules and consequently have 

 a hairy texture, greatly changing the general appearance of 

 the bird. " A few feathers of this kind have been found in 

 Hawks and Gulls, and in the case of a Parra (a bird which 



