PROBLEM OF ORIGIN OF SPECIES 53 



equally manifest that the process of reduction is making more 

 rapid progress in the fore part of the wing and along its lower edge 

 than elsewhere. There can be no mistake here as to the direction 

 in which the phenomena are to be read. The direction is as certain 

 as that the adult male stands in advance of the adult female, and 

 still more in advance of the young bird. The significance of the case 

 lies mainly in the fact that it is not an isolated or exceptional one. 

 Many other species tell more or less perfectly the same story. 



A parallel case, only carried still farther in the same direction, is 

 found in the mourning dove (Zenaidura). The adult male and fe- 

 male differ but slightly, each having only about a dozen checkers 

 visible on each side. These are confined to the scapulars, and to a 

 few feathers at the posterior upper edge of the wing. In the young, 

 they are more numerous, but less so than in the young passenger 

 pigeon. The middle and fore parts of the wing in the adult have no 

 visible checkers, but a few concealed ones which may be seen on 

 lifting the overlying feathers. These concealed checkers, and other 

 differences between old and young, show that the species had its 

 origin in a checkered stock, and that its history has been analogous 

 to that of the passenger pigeon. 



The white-winged pigeon (Melopelia leucoptera) is a most instruct- 

 ive form. Although a much more highly accomplished bird in the 

 arts of display of form, feathers, and voice, than the mourning dove, 

 it has suffered a complete effacement of the checkers it once pos- 

 sessed in common with other members of the family. Indubitable 

 proof of this is to be seen in the Juvenal feathers, which, in some 

 cases, exhibit a few pale vestigial spots in the last two rows of long 

 coverts, at points where the checkers are usually best developed in 

 checkered species. Another striking proof is to be found in the 

 coverts and scapulars of the adult bird, where we find, on lifting 

 the feathers, distinctly outlined areas, corresponding in shape and 

 position with reduced checkers, but from which the black pigment 

 has disappeared. These vestigial outlines, structurally defined, 

 were first noticed in a female bird of a dark shade. 1 The outlines 

 were more perfect than in lighter birds obtained from Arizona and 

 California. 



Similar vestiges are present in the mourning dove, and here their 

 identification as marks formerly filled out with black pigment is 

 freed from every shadow of doubt by checkers in all stages of oblitera- 

 tion. 



The large wood pigeon (C. palumbus) of Europe has departed 



still more widely from the turtle-dove type, having lost all its black 



spots except a few in the neck patches, which have retreated so far 



from the tips of the feathers as to be concealed. The gray plumage 



1 Captured in Jamaica by Dr. Humphreys. 



