No. 631] 



NOTES AND LITERATURE 



L89 



pendent studies of wild rock-pigeons obtained from the "Caves 

 of Cromarty, Scotland." He found that not all the wild pigeons 

 of this locality are of the simple two-wing-bar type, but that 

 part of them show a different pattern known as "chequered." 

 In these also the two black wing-bars can be observed, but they 

 are rendered less conspicuous by the occurrence of other black 

 spots scattered over other parts of the wing, giving the whole a 

 chequered appearance. The wing-bars are due to the occurrence 

 of a black spot on the tip or below the tip of each of two rows of 

 feathers that lie across the wing when it is folded. In chequered 

 birds other rows of feathers bear spots but the spots fall less 

 regularly and obviously into rows, so that the pattern is more 

 like that of a chequer-board. Further in young birds Whitman 

 observed that practically all the wing feathers may bear spots, 

 although in the later plumage some of the spots may disappear. 

 He concluded that this condition was the primitive one, rather 

 than the two-wing-bar type which Darwin regarded as primitive. 

 This conclusion seems well founded since the chequered type is 

 thus seen to be less specialized in form and earlier in ontogeny. 

 So far Whitman's work supported Darwin's general evolutionary 

 ideas, merely improving a detail in one of his illustrations, and 

 showing that there still exists among wild pigeons a pattern yet 

 more primitive than the one which Darwin had taken as the point 

 of evolutionary departure. But Whitman now extended his in- 

 vi'M Virions to other species of pigeons and finally to those of 

 the entire world to see if he could work out more fully the evo- 

 lutionary history of plumage patterns in the group. As a result 

 of these studies he reached conclusions which did not enter into 

 Darwin's scheme of evolution. The most important of these is 

 known by the name of "orthogenesis." This is the idea that 

 evolution through natural selection does not result simply from 

 the selection of chance variations, that variations do not occur in 

 all directions but only in particular directions in straight lines 

 from the point of departure, hence the name orthogenesis. Whit- 

 man's study of the plumage patterns of pigeons is probably the 

 most extensive, as it is the most recent, of the studies of a group 

 of animals made in the light of this principle, but to the general 

 body of biologists free from bias for any particular theory it 

 will scarcely be more convincing than its predecessors. It is 

 possible to arrange any group of related organisms in a graded 

 series and to assume that they have been evolved by orderly de- 

 velopment, from one end of the series (either end) to the other; 



