EXTERNAL FEATURES 37 



many feathers are blue or white there are no blue or white 

 pigments in birds. 



The study of feather pigments shows very clearly that 

 one and the same pigment may result in very diverse colours. 

 This has been convincingly shown by Orren Lloyd-Jones 

 (1915) in the case of the tumbler pigeon. There is a red- 

 brown pigment of the melanin series, which gives a red 

 colour when it occurs in spherical granules of about 0*3 ju, 

 in diameter, "plum" colour when the granules are 2/x or 

 more in diameter, and yellow colour when the division is 

 so fine that its granule form cannot be determined. The 

 difference in colour is a question of the microscopical 

 dimensions of the granules of one and the same pigment. 



There is in the same pigeon another melanin pigment 

 which under different conditions yields black, dun, blue, 

 and silver colours ! In black birds the pigment occurs as 

 spheres o'^fx in diameter, or as rods i[x in length, some- 

 times mingled, sometimes separated. In dun birds the 

 granules are invariably spherical and about o'3/x in diameter. 

 " Blue " is due to a peculiar clumping and distribution of 

 the pigment in the barbules, and " silver " is due to a 

 similar distribution, but associated with a pitting and 

 roughening of the barbule surface. 



According to Lloyd- Jones, all the fundamental self- 

 colours of tumbler pigeons can be accounted for by the 

 interaction of four hereditary factors : R, for red pigment ; 

 B, for black pigment ; I, for intensity, increasing the 

 amount of pigment produced ; and S, for spreading, which 

 apparently means stopping the clumping of the pigment in 

 the middle of the barbule cells. 



This investigation is interesting, (i) in showing how 

 very different colours may result from the same pigment 

 (the red or the black), and (2) in illustrating how four 

 hereditary items or factors may by different shufflings of 

 the cards yield diverse final results. 



Physical or structural coloration is illustrated in two 

 ways, (a) In some cases the colour changes with the 

 changing light and the position of our eye, the changes of 



