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- 



