240 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



not in the very slightest degree affected by the pig- 

 ment alone ; but, curiously enough, particolored birds 

 did react, the brown parts of the feathers became 

 distinctly lighter in hue. It is a fatty substance (trio- 

 lein) which appears to convey the pigment, and to pro- 

 duce thus a changing of the color from yellow to red ; 

 and further experiments were made with other birds, 

 showing that it is not only canaries which are influenced 

 by their food in this way. Some white fowls, belong- 

 ing to a special breed, showed traces of yellow among 

 the feathers after feeding with cayenne ; but in this 

 case there were not racial but individual differences in 

 susceptibility, for all the specimens of birds experi- 

 mented with did not react to the stimulus. 



"A similar series of experiments was made with 

 some other colors : it was found with carmine that the 

 yellow color was destroyed and the birds became white. 

 This unexpected effect is explained by the fact that a 

 mixture of violet and yellow produces white. The fact 

 that the fatty constituent, triolein, plays the chief part 

 in the coloring of the feathers may perhaps help to ex- 

 plain the very singular fact that the Amazon parrots 

 change from green to yellow when fed upon the fat of 

 certain fishes. 



"With regard to the white fowls referred to, the 

 experiments made by Dr. Sauermann were particularly 

 interesting. The interest lies in the fact that the pig- 

 ment was not absorbed equally by all the feathers ; 

 only special tracts were affected ; the breast feathers, 

 for instance, became red, while the head remained 

 white. It is therefore quite credible that in a state of 

 nature partial alteration of color may be produced by 

 a change of diet." 



In a chapter of Dr. Beddard's book relating to pro- 



