GENESIS OF FAULT-BARS IN FEATHERS. 357 



the production of all fault-bars and of the light fundamental bars ; 

 while the better normal nutrition of the day and of the first part 

 of the night is associated with the production of the dark funda- 

 mental bars. 



The large light and dark transverse bars with which we have 

 all long been familiar in Plymouth Rock fowls, in hawks, in jays, 

 etc., are of course not the light and dark fundamental bars with 

 which this work deals, and are not each the growth of a day or 

 night ; it is perfectly evident that in them a single broad black or 

 white bar may include the growth of two, three or more days. 

 But even these broad bands of white and black may later be found 

 to bear secondary or derivable relations to the fundamental bars. 



The alternating light and dark fundamental bars are only rarely 

 seen in their fullest development, i. <?., as well defined alternating 

 bands of lighter and darker color. Experience Would indicate 

 that they are found to best advantage in pale feathers rather than 

 in those with a superabundance of pigment. The separation of 

 the feather into the faint defective lines, and the broader well- 

 grown areas is, however, easily found in all feathers. The light 

 and dark fundamental bars are shown in PI. XIII., Figs. 15-16. 



Discussion. 



It is then through such a mechanism as has been described 

 in the foregoing pages that the melanin pigment of feathers comes 

 to be laid down in alternating light and dark transverse bars. 



In concluding the presentation of this subject, I shall attempt 

 no extended discussion of the relation of this to similar work, nor 

 of its relation to general biological problems. A few statements 

 of this import, touching upon that part of this work which deals 

 with color-formation may, however, not be too wide of the mark. 



It can be said that thus far those who have essayed to carry 

 the puzzling facts and phenomena of animal coloration from the 

 dark fields of heredity into the proved and clarified domains of 

 physiology have met with only partial success at best. The efforts 

 of Graf and of Loeb in this direction were mentioned in my 

 introductory statement as among the most successful. 



Graf's contribution is that in leeches certain tissues are found 

 to offer greater resistance than others to the migrating " excre- 



