60 ORTHOGENETIC EVOLUTION IN PIGEONS. 



reduction is the same for both types, but a slight difference in detail is superadded 

 for the chequered type. The light edge, which is an important and permanent 

 feature in the spotted type, is very narrow and transitory in the chequered type, 

 disappearing with the molt of the juvenal feathers. 



Within this narrow light (pale brown) edge appears a zone of gray that usually 

 enlarges at the middle and extends inward in the form of a wedge, or as a mesial 

 streak, dividing the dark spot more or less deeply into two lateral chequers with 

 pointed tips. The gray area, which arises and enlarges at the expense of the black 

 spot, varies greatly in form and extent in different feathers and in different indi- 

 viduals. It may be wholly absent in some feathers, the black spot remaining undi- 

 vided, as in the turtle-dove; it may take the form of a narrow crescent more or less 

 thickened at the middle; it may have the form of a triangular spot, as in the guinea- 

 pigeon (Columba guinea) of Africa, 14 or be more obtusely pointed, as in the spotted 

 pigeon (Columba maculosa) of South America (pi. 18); it may expand over the 

 whole width of the feather and enlarge mesially so as to deeply or completely divide 

 the black; the mesial streak may expand equally on both sides of the shaft until 

 the two spots are left as edge streaks, or unequally, leaving more or less of one spot 

 than of the other. 



The mesial extension of the reduction process, which prevails in the chequered 

 type and is its chief peculiarity, is not after all an exclusive distinction. A light 

 mesial streak dividing a central dark spot may and actually does occur in the breast 

 feathers of some of the young of the European turtle-dove; and thus we see that 

 both primary characters — the single central spot and the two bilateral spots — may 

 occur together in each type. 



I have seen the same mesial streak in the juvenal upper wing-coverts of Leptop- 

 tila (pi. 43). It occurs also in the young robin (see pis. 56, 57), where it closely 

 simulates conditions presented in domestic pigeons. It is a permanent condition 

 in a large number of birds, the female jungle-fowl being an example. 



We have seen that the process of reduction tends to sweep the whole surface 

 and in the same general direction. The conclusion supported by comparative study 

 admits of experimental confirmation. We may take pigeons of the two-barred type, 

 and try to advance from this condition to that of the chequered type, by selecting 

 in each generation birds with the widest bars, and especially any that may have a 

 trace of a third bar. This I have tried 15 continuously for six years and with several 

 different stocks. I have not been able to establish a third bar, or to extend chequers 

 in front of the vestigial third bar, which is often found. With pure-bred birds, not 

 allowed to mingle with chequered birds, I believe it is impossible to advance from bars 

 to the chequered state. With chequered pigeons, on theother hand, it is fairly easy to 

 advance in the opposite direction, gradually clearing the field and leaving two bars. 

 The process has been carried to the point of completely eliminating the bars. 



From these considerations we can readily understand why the stock-dove, 16 

 which has, at least in many cases, a vestigial third bar, quite like that in domestic 



14 See Chapter IX and illustrations. The present discussion of the "guinea-mark" — or triangular white tip of the 

 feather — may serve as an introduction to the "guinea-mark" mutations, which are described and illustrated in Chapter 

 IX. It is partly for this reason that some of the illustrations dialing with this character are placed with the references 

 to them in the present chapter. — El). 



15 As noted in Chapter II. — Ed. 



'• Columba cetiax, discussed and figured in Chapter III. — Ed. 



