58 ORTHOGENETIC EVOLUTION IN PIGEONS. 



not mere blotches of pigment added to an undifferentiated surface, but structu- 

 rally defined areas, which are the skeleton outlines or blank forms once filled out 

 with pigment. That they should persist for a time after the pigment has ceased 

 to appear is not surprising. 



The process underlying the evolution of color-patterns, so far as illustrated in 

 the species referred to, only takes us back to the chequered condition as a starting- 

 point. How pigeons came into possession of chequers is quite as interesting a 

 question as how they are losing them. This I will briefly indicate, in order to make 

 clear how the same process of evolution reaches far back to the primeval history 

 of the pigeon group and, in fact, to that of the whole class of birds. 



As before remarked, we find in the turtle-doves of the Old World a pattern which 

 seems to have been the aboriginal pattern for the pigeons and other birds as well. 

 With this pattern as an archetype it is possible to get an orientation of the whole 

 field of avian patterns and to thread our way through what before seemed an impene- 

 trable maze of multifarious variations, with no discoverable beginning or end or order. 



The pattern is very simple, each feather having a dark center and a light edge. 

 This simple unit or elementary pattern, repeating itself with little or no variation 

 in each feather, results in a uniformly spotted pattern for the bird as a whole. This 

 pattern is well-preserved in the oriental turtle-dove from Japan; but in the adult 

 it has already suffered reduction in the head, neck, and breast regions, .only a 

 patch being left on each side of the neck. In the young (in color, pi. 2, Vol. II) 

 the pattern is carried out on these regions, but less strongly than elsewhere, and 

 without any distinct patch on the neck. On the wings and back the Juvenal pattern 

 is the same as the adult. The young bird, then, gives no intimation of an earlier 

 pattern, but in striking directly into the general 'surface-pattern of the adult bears 

 witness to its primeval character. 



In birds that have made considerable departure from this type the young 

 generally repeat the pattern in their first feathers and exchange it for the later- 

 acquired pattern at the first molt. The adults may retain the original marking 

 in some regions while advancing beyond it in other parts, and in such cases we 

 get the clearest evidence in transitional phases of the direction of modification. 

 Even in the widest departures, where every spot has vanished in the adult plumage, 

 the young bird frequently exhibits more or less perfect traces of the old marking 

 and sometimes requires several molts to reach its mature condition. 



Without farther digressing into the general field of color-patterns, it remains 

 to explain the genetic connection between the chequered and the spotted type, 

 and to show how both types originated in and have been diversified by the same 

 general process of reduction of pigment. 



The light apical edge of the feather, by the presence of which the central field 

 becomes defined as a spot, represents a first step in this direction. That this is 

 the correct interpretation is conclusively shown in the turtle-doves and their nearer 

 allies. Taking the oriental turtle-dove as the least modified in this direction, and 

 comparing it with the European species Turtur turtur (color, pi. 2, Vol. II; wing, 

 pi. 22, Chapter V), we see at once that a distinct advance has been made the 

 light edge is wider and the spot correspondingly reduced, becoming more sharply 

 pointed. Tn another species, the Surate turtle-dove (Spilopelia suratensis), the 

 reduction is carried still farther (pi. 24, Vol. II), having only a mesial streak. In 



