50 ORTHOGENETIC EVOLUTION IN PIGEONS. 



prevalence of brown among the wild species of pigeons, and, further, in the fact 

 that in all species with a gray plumage in the adult state the juvenal feathers are 

 always suffused with brown. 



The first ancestors of the pigeons, or of the pigeons together with all other 

 birds, may have been of a single unspotted 2 brown color. But as color-patterns 

 are found quite generally distributed among the lower orders of animals, it is hardly 

 probable that the avian branch came into existence entirely unprepared for the 

 decorative supremacy it was destined to enjoy. 



Be this as it may, it can be made very clear that the rock-pigeons are not those 

 first ancestors, nor yet their nearest relatives among the pigeons. The rock-pigeons 

 represent a terminal twig in the avian genealogical tree. The main stem of the 

 pigeon branch, if I read correctly the testimony of color-patterns, is represented 

 most nearly by the turtle-doves (Turtur orientalis and T. turtur) of the Old World. 

 The original turtle-dove pattern, in which all or most of the feathers were similarly 

 differentiated into a dark center and a light edge, seems to have been a veiy general 

 if not a universal avian pattern. This pattern certainly preceded the chequered 

 type of the rock-pigeon (Colwnba affinis), and it is possible still to find connecting 

 types — types in which the turtle pattern coexists with the chequered pattern— 

 the latter coming in to replace or supersede the former. Examples are to be seen 

 in the bronze-winged pigeon (Phaps chalcoptera) of Australia, the Florida ground- 

 dove (Chamcepelia passerina) , and some others. The genetic connections between 

 these two patterns belonging to two distinct families have been considered in 

 Chapter II. 



The main problem to be dealt with in this paper concerns the closer relation of 

 two patterns differing at most only in specific rank. That the two patterns are 

 connected by direct lineal derivation is not questioned. But what is the direction 

 and what the mode of derivation? The problem is only one of thousands of similar 

 problems; but if it admits of a decisive solution it will furnish answers to a few 

 questions of leading interest at the present time. 



The wing-bars of the rock-pigeon represent a specific character with a history 

 that can be clearly read in both the wild and the domesticated forms. The chief 

 transitional phases are to be seen in almost any flock of mixed breeds. The direc- 

 tion of evolution can be determined in two ways: (1) by comparison of individuals, 

 young and old, in each of the two species and in domestic races; and (2) by com- 

 parison of the different wild species that are marked with spots and bars in various 

 stages of evolution or decline. The conclusions arrived at through comparative 

 observation can then be tested by experiments in breeding, cross-breeding, etc. 



In tracing the origin and genesis of a single character we meet the leading 

 questions in the evolution of species. First and foremost, the question as to the 

 nature of the initial stages. Did the character arise as a variation de novo, or as a 

 progressive modification of a preexisting character? If de novo, did it spring 

 suddenly forth, with some decisive advantage in the struggle for existence? or did 

 it appear as one of many minute changes, and by some happy chance get a start 

 that gave it the lead in future development? In other words, did it begin as a 

 discontinuous variation, sport, or mutation? or did it arise cumulatively, as a con- 

 tinuous development? If it originated by modification of an earlier character, was 



2 Later Professor Whitman questioned this possibility much more seriously than is done here. — Ed. 



