THE PROBLEM OF THE ORIGIN OF SPECIES. 



27 



Text-figure 7. — Cheq- 

 uers of adult male pass- 

 enger- pigeon. Tod a 

 del., Nov. 1903. x0.5. 



In birds taken at random, I count in the left wing and scapulars 90 chequers 

 on a juvenal, 51 in an adult female, and 25 in an adult male. This is pretty 

 conclusive evidence that chequers are, or have been, disappearing in the species. 

 Not only the number but also the size of the chequers has been reduced. In the 

 female the chequers are for the most part two or more times as large as in the 

 male. The reduction in both respects has been greater in the anterior than in the 

 posterior half of the wing, and greater 

 along the lower edge than in the middle 

 and back regions. 



In this species we may recognize at first 

 sight the homologues of the rock-pigeon bars. 

 On the secondaries of the female we find the 

 homologue of the posterior bar, and on the 

 first row of long coverts the homologue of the 

 anterior bar. The latter is scarcely recogniz- 

 able as a bar, for we see only 5 or 6 chequers 

 in the upper half of the row, the lower half 

 being without chequers. Nevertheless, this 

 row represents, so far as it goes, the elements 

 of a bar which is already too far gone to have 

 even a chance to attain the finish of a per- 

 fect bar. 9 



On the secondaries the chequers fall into 

 juxtaposition, forming a continuous bar, with 

 an irregular posterior outline, which indicates 

 that the chequers have been unevenly re- 

 duced from behind. It is a rudely finished 

 bar which has sunk below the horizon of utility, 

 if it was ever above it, and is now facing ulti- 

 mate eff acement. The reduction has advanced 

 further in the male, with no improvement 

 towards regularity of outline. Here it becomes 

 quite certain that effacement advances 

 from all sides, leaving but a small remnant 

 of a bar confined to two or three feathers. 



Glancing at the wing as a whole, in 

 both young and old, it is plain that the 

 process of obliteration is in progress over 

 the entire chequered area. The elongated, 

 sharp-pointed marks of the earlier pattern have rounded tips in the adult; the 

 posterior bar is roughly emarginated; the number of chequers is reduced by half 

 or more; and some of the remaining ones are but little more than mere dots. It 

 is also 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. 



'' In the young, the chequers of this row are more numerous and much more sharply pointed at the ends. In 

 both respects the juvenal pattern approaches more nearly a condition of general uniformity. 



