152 ORTHOGENETIC EVOLUTION IN PIGEONS. 



The first stages of the variation are usually too minute to attract attention. 

 They are preparatory stages that lead sooner or later to visible phases. The visible 

 phase, if cultivated by artificial or natural selection, can be developed to almost 

 any extent compatible with the existence of the organism. The higher stages may 

 differ more and more in some structural element, so that the form presented changes 

 progressively, the final form being very different from the initial form. 



FUNDAMENTAL BARS. 



A still older character than the dark center of the turtle-dove feather is seen in 

 the cross-bars, or fundamental bars, that appear to mark all feathers of all species 

 in birds. These bars were first noticed in pigeons in the summer of 1903, and were 

 soon found to be common to all species of pigeons and birds in general. From these 

 fundamental feather-bars, or their secondary derivatives, a multitude of specific 

 characters have been evolved by gradual modification. The continuity in the evo- 

 lution of some of these characters can be experimentally demonstrated. The little 

 diamond-dove {Geopelia cuneata) of Australia owes its small white spots (two to 

 each feather) to these bars. The transitional stages connecting the spots with the 

 bars are not wholly given in passing from the juvenal to the adult plumage. But 

 if we pluck a few of the juvenal feathers at suitable intervals, their places will be 

 filled by new feathers of different ages, and in this way we may get the stages 

 intermediate between the bars of the young and the spots of the adult. Thus we 

 see that the adult pattern, which normally appears to come in as a striking mutation, 

 by a single jump, is only an end-stage in a continuous process of differentiation. 

 So it is everywhere. Suppression of stages in ontogeny looks like saltations; but 

 whenever we can get at the history of the character, we find the continuity comes 

 to light. 



The characteristic secondary cross-bars of many races of the common fowl, 

 pheasants, guinea-fowl, ducks, woodpeckers, etc., have been molded more or less 

 directly out of or upon these fundamental cross-bars, which everywhere underlie 

 other color-marks, even the universal avian character already described, and its 

 latest derivatives. 



The discovery of the universality of this character — fundamental bars — and its 

 far-reaching significance as an initial foundation for numerous specific characters, 

 suggested the need of a thorough investigation to determine its nature and mode of 

 development. This task has been undertaken and brought to a conclusion by Mr. 

 Oscar Riddle, who has not only reached a physiological explanation of the character, 

 but has fully confirmed and established the following anticipation I ventured to 

 make in 1903: 



Allowing that the feathers of the common pigeon get their full length in 4 weeks, and 

 that the terminal half of the feather (on which the bars are distinct enough to be counted) 

 is formed in 14 days, it is found that the number of bars corresponds nearly to the number 

 of days of growth. If this be so, then the bars would be zones of daily growth (light = 

 day; dark= night, or vice versa). n 



19 The preceding paragraphs form part of the abstract of a lecture delivered before the Wisconsin Natural History 

 Society on December 20, 1906, and published in the proceedings of that society in 1907. For the introduction, and 

 the remainder of the published abstract of that lecture, see the beginning of the following chapter. — Ed. 



