THE TURTLE-DOVE PATTERN IN THE PHYLOGENY OF PIGEONS. 83 



Now, nature performs just this experiment on the young of many species of 

 pigeons, both wild and domesticated. By varying slightly the time of delay, and by 

 operating on many different patterns, and in several regions of each pattern, nature 

 offers a profusion of demonstrations, showing that it is not mutation, but the "law 

 of genetic continuity" — one continuous process of development that underlies and 

 connects the phenomena of colors and patterns in successive plumages. The intervals 

 between stages are not periods of rest, but of active progress in transitional elabora- 

 tion. The visible stages and the invisible flow of transitional latent phases together 

 form a continuum, with some analogy, though only highly fanciful, to the rhizome 

 with its visible nodal growths arranged in serial order, but appearing as isolated gene- 

 rations, because the internodal connections lie hidden beneath the surface of the soil. 



One of these natural experiments is realized in this narrow "oblique tract" 

 which, running parallel with the bones of the forearm, crosses the wing obliquely 

 in the region of the anterior or lesser coverts. In this line, for some reason yet to 

 be ascertained, two or three rows of coverts appear a week or two later than the 

 other coverts of the wing, the time varying for different species. 



In all cases where there are considerable differences in color or pattern between 

 the first and the second plumage, these belated feathers will appear in more or 

 less conspicuous transitional conditions. When, for example, in the domesticated 

 pigeons, a clear black is to succeed a blackish-red, black will predominate in the 

 streak and thus render it quite distinct. But the black here, as a rule, will not be 

 as strong as it is to be in the second plumage; and as the first molt leaves these 

 feathers intact until after that plumage appears, the streak will still be recognizable, 

 but in this case from deficiency rather than excess of black. 



Black pigeons sometimes produce pale or dusky red young, as a pair of black 

 barbs has repeatedly done during the past several years. The streak in these young 

 is a shade or two darker than other parts, and anticipates a still darker red in the 

 second plumage. 



In young pigeons of the second or double-barred varieties the colors are com- 

 monly paler than in the adult, and the gray ground-color is suffused with brown, 

 which disappears in the second plumage. In such birds the streak shows a stronger 

 black and a clearer gray, approximating more or less closely the adult colors. 



It is not in the domestic pigeons, however, that we get the more striking and 

 instructive pictures of progressive transitional change in this region of the wing. 

 For such views we must go to some of the wild species, in which the adult pattern 

 departs more widely from the juvenal pattern. It is in such species, where the gap 

 between patterns seems so wide and so abrupt as to suggest a sudden and whole- 

 sale mutation, that we find in this tract of feathers the most complete demonstra- 

 tions of continuity through intergraded or transitional phases. 



For the purpose in view we must of course have young birds accessible for con- 

 venient observation from day to day, and such material is obtainable only from 

 species that will breed in captivity. It is for this reason that the phenomena in 

 question have hitherto escaped the attention of naturalists. 



The little ground-doves of the genus Geopelia, found principally in Australia, 

 answer the requirements named and furnish the best demonstrations I have thus 

 far discovered. 34 



"Recapitulation of the light apical edge of G. Iranquilla and of the bars which precede the white spots or dots of 

 G. cuneala are described and figured at the close of Chapter X. For the recapitulation of still other characters in 

 Geopelia see the following section.- — Ed. 



