150 PROBLEMS OF GENETICS 



Guillemot ( Uria troile) which has a white line round the eyes and 

 at the sides of the head where the normal has no such mark; but 

 this line is formed in a very special place, the groove joining the 

 eye to the ear, whereas the feathers of the nuchal crescent are 

 not ostensibly distinguished from those adjacent. 5 



The transposition of the brown and the grey on the back and 

 front of the neck also constitutes a very remarkable difference. 

 If either grey or brown depends on a factor then it must be sup- 

 posed that auratus has one of these factors and cafer the other. 



From these several considerations it is quite clear that if 

 auratus and cafer are modifications of the same type produced 

 by presence or absence of factors, several independent elements 

 must be concerned, and to unravel their inter-relations would be 

 most difficult even if it were possible to breed the types under ob- 

 servation, which is of course quite beyond present possibilities. 



The distribution of the two is as follows. On the east side of 

 the Continent C. auratus, relatively pure, occupies the whole of 

 Canada and the States from the North to Galveston. Westward 

 it extends across the whole continent in the more northern 

 region to Alaska, but in its pure form it only reaches down the 

 Pacific coast to about the northern border of British Columbia. 

 Its southern and western limit is thus roughly a line drawn from 

 north of Vancouver, southeast to North Dakota and then south 

 to Galveston. C. cafer in the comparatively pure form inhabits 

 Mexico, Arizona, California (except Lower California and the 

 opposite coast), central and western Nevada, Utah, Oregon, and 

 is bounded on the east by a line drawn from the Pacific south of 

 Washington, south and eastward through Colorado to the mouth 



8 This variety is spoken of as the Ringed Guillemot and is sometimes regarded 

 as a distinct species to which the name ringvia was given by Brunnich. In sup- 

 port of this view Dr. William Brewster, to whom I am indebted for much assist- 

 ance in regard to the variation of birds, called my attention to observations of his 

 own and also of Maynard's, that the ringed birds were sometimes mated together, 

 though in a small minority (see Brewster, Proc. Boston Soc. N. H., XXII, 1883, p. 

 410). It would however be possible to produce many instances of varieties mated 

 together though surrounded by a typical population (e. g., two varying Black- 

 birds, Zoologist, p. 2765; two varying Nightjars, ibid., p. 5278). I am inclined to 

 believe that in nature matings between brothers and sisters are frequent in many 

 species of animals, and that the production of sporadically varying colonies is 

 thus greatly assisted. 



