172 BULLETIN : MUSEUM OF COMPAIUTIVE ZOOLOGY. 



arranged parallel with the axis of the barbule (Plate 5, Figs. 26, 27), 

 a condition for which I luive no explanation. 



The variations in pattern exhibited by a single feather, in the form 

 of bars, spots, etc., are easily correlated with variations in the distri- 

 bution of pigment in the corresponding regions of the feather germ. 



That the distribution of lipochrome pigments to the feather funda- 

 ment takes place at about the same stages in the development of the 

 feather as that of the melanins, seems certain. Tiie germs of yellow 

 feathers from the canary and the nonpareil show a yellow color which 

 corresponds in position to the dark color of feather germs pigmented 

 with melanin. 



VI. Change of Color without Molt. 



The changes in color claimed by many writers to occur without molt 

 may be grouped under two heads : (1) the destructive, and (2) the con- 

 structive. Under destructive changes are included the results of 

 abrasion and physical disintegration. Constructive changes include 

 supposed regeneration and rearrangement of pigment. 



For a review of the general literature of change of color without molt, 

 the reader is referred to Allen ('96). More recently Meerwarth ('98) 

 has claimed that change of color without molt occurs in the tail- 

 feathers of cei'tain Brazilian Raptores. He describes variations in color 

 pattern that he has observed in material consisting mostly of skins. 

 His paper gives no satisfying evidence that the changes alleged may 

 not have taken place through irregular molting. Furthermore, he does 

 not offer any explanation of the process of change. 



Descriptions of repigmentation have been mostly pure speculation. 

 Within a few years the following remarkable explanation of the pig- 

 mentation of the feather has been given by Keeler ('93) : " Pigment is 

 a definite chemical substance which travels through the various l)r:inches 

 of the feather, advancing farthest and most rapidly along the lines of 

 least resistance and accumulating in masses where the resistance is 

 greatest. Now the pigment cells must reach the various parts of the 

 feather by way of the shaft, and we should a priori expect to find tliat 

 the resistance would be least down the shaft. It might spread out a 

 very short distance on the barbs, but the main tendency would be 

 towards the tip. This would produce a streaked feather as the most 

 primitive form." 



Still more recently Birtwell (:00), in arguing for change of color with- 



