56 



ORTHOGENETIC EVOLUTION IN PIGEONS. 



It is here that we may with some reason suspect the intervention of natural 

 selection. It would, in this case, come in, not as a primary factor to originate a 

 new character, but adventitiously, by invitation, as it were, of favoring predetermi- 

 nations and environmental conditions. The process of deletion of the chequers, 

 traveling backward and upward, would tend directly to clearing the field in a way 

 to leave a few chequers just where we now find them. The ornamental value of 

 these few chequers and their utility as recognition-marks would obviously be 

 enhanced by their isolation in a plain ground, just as a few trees, concealed in a 

 large forest, become conspicuous when left standing alone. These chequers, being 

 on the larger feathers, would have the advantage of size, and so their preeminence, 



TEXT-FIGURE 13. Adult male Nesopelia galapagoensis. Natural size. Hayashi del., June 1907, from 

 a skin loaned by the U. S. National Museum. 



1. Three or four irregular rows of the anterior lesser coverts have lost the chequers. 



2. Following these are one or two rows of lesser coverts with vestigial chequers, mostly concealed. 



3. Then follow five rows of coverts bearing chequers. Of these five rows, one could be reckoned with the lesser 

 coverts, three as median, and one as long or larger. Each feather usually bears two spots, the outer spot the 

 larger; passing from the inside outward, the spot on the inner web becomes rapidly smaller until at least it is only a 

 marginal streak, and then the outermost feathers bear only one chequer on the outer web. The chequers are mostly 

 rounded distally. The dotted part of the feathers is brown, and the white spaces between spots and behind them are 

 left as they appear. 



4. The scapulars are double-spotted. 



5. The tertials are double-spotted, but the outer spot becomes smaller passing downward. 



6. On the inner tertials there is asymmetry in position of the two spots, as in mourning-dove. 



7. The secondaries have black spots, so that here we get the essentials of a posterior wing-band. 



8. Many feathers of the back, between the scapulars, are spotted black. Nesopelia is more primitive than the 

 mourning-dove in the number of spots, but has white between spots, which makes them much more conspicuous than 

 in the mourning-dove. 



attained without the aid of natural selection, would be an open door through 

 which it might enter and contribute to their improvement. The part possibly 

 taken, however, could at most be but a late and inconsiderable share of the total 

 achievement summed up in these spots; and the course of events in at least one 

 of the allied forms (Melopelia) indicates that these marks are destined to be 

 washed out. 



The question here raised is one of general interest, and it could probably be 

 settled by an extended study of the marks in the mourning-dove and a comparison 

 of all the Zenaidinae. Nesopelia galapagoensis is the only one of this family which 

 is fairly evenly chequered over the whole wing and scapulars (text-fig. 13), and 

 this condition stamps it as the oldest member. Melopelia, with its two species, 



