Fatlo on Feathers — their Decoloration. 175 



variegated colours on their breasts and bellies, eitlier from the 

 materials of their nests, or from that of the hole which they 

 inhabit. Thus in spring I have seen the Parus borealis almost 

 completely red on its lower surfaces. Lastly, an external 

 colour, attaching especially to the throat, sometimes arises from 

 coloured food, as when the throat and breast of the nutcracker 

 assumes a dark rust colour wheu, during the frosts, it comes 

 down to the valleys and feeds greedily on the hazel-nuts, in 

 which it delights.-" 



I OXCLUSIOXS. 



u I have pointed out the principal agents which modify 

 feathers, and their mode of action. I have also shown how 

 these same agents can sometimes produce varied effects under 

 different conditions ; and lastly, I have shown how a certain 

 equilibrium between external and internal condition is necessaiy 

 to keep the coloration of a species within typical limits.-" 



" I cannot assert that I have foreseen all the divers causes of 

 natural or accidental variation, nor can I pretend to have sub- 

 mitted to examination all the different feathers which birds may 

 present. I have passed by some modifications purely orna- 

 mental ; but I think I have investigated the most natural and 

 ordinary forms, and that new phenomonon may easily find their 

 explanation upon the principles thus disclosed." 



%. H< >!< % H- >^ ^ 



w My explanation comprehends not only the variation of 

 colours more or less striking according to locality and climate, 

 but also the formation of race and varieties according to the 

 prevalence of different conditions of moisture, temperature, and 

 inclination." 



