268 Stejneger, Isolation vs. Natural Selection. [July 



tip as the original forms, from which the western birds were derived 

 by a reduction of the spots on the distal end of the wing feathers. 

 Just the opposite is the case. 



Speaking of the birds in general it may be said that there is an 

 evolutionary tendency towards a uniformly colored plumage as 

 distinct from the dark-and-light (not black-and-white) spotted or 

 striped plumage. This accomplished, the further development 

 tends towards pure white, either as a uniform white plumage or one 

 spotted with pure white. The white spots are not produced by 

 any white coloring matter, on the contrary, they are due to total 

 absence of pigment. Generally speaking they are a degradational 

 development, they betray a structural weakening of the feather. 

 As for the wing feathers this process usually begins at the base of 

 the feathers, a basal white spot being present in many birds although 

 absolutely hidden. A further development of it is the white wing 

 'speculum' of so many birds. Often it develops still further, but 

 seldom extends to the very tip, unless the whole plumage tends to 

 uniform white, and it is mostly confined to the part of the feather 

 covered by the superposed portion of the next feather. Where the 

 distally increasing whitening of the feather — and we are now 

 coming to the case of the spotted-winged Woodpeckers — does not 

 proceed as a solid white area, but as a series of white spots, the 

 remaining black cross bars have the function of stays or braces. 

 It is easy to understand then why the white spots appear first on 

 the inner webs of the secondaries and last on the outer webs of 

 the primaries or at the tips which extend beyond the plane of the 

 wing when spread for flight. 



That this degradation of the feathers in the birds of northern 

 affinities as a rule increases towards the colder regions there can be 

 no doubt, and as for the spotted woodpeckers of the genus Dryo- 

 bates I need only refer to the Siberian and Kamchatkan forms for 

 proof. That this increase in the size and number of white areas 

 is not directly attributable to the effect of the cold seems evident 

 (see my Ornith. Res. Expl. Kamtch., 1885, pp. 343-344). Barrett- 

 Hamilton, not long ago, has suggested a theory connecting the 

 absence of pigment with the accumulation of fat in the under- 

 lying tissues of the body, but in the woodpeckers at least I fancy it 

 would be difficult to establish such a connection. It seems to me, 



