22 THE STORY OF THE BIRDS 



Birds, it is no longer the same when the light 

 is reflected from these feathers, each of which 

 performs the office of reflector. Then it is that 

 the aspect of the emerald, the ruby and so on, 

 varying with the utmost diversity under the 

 incidences of the rays which strike them, is 

 given out by the molecular arrangement of the 

 barbules. It is thus that the gorget of many 

 Humming-Birds takes all the hues of green, and 

 then the brightest and most uniformly golden 

 tints, down to intense velvet-black, or, on the 

 contrary, that of ruby, which darts forth pencils 

 of light, or passes from reddish orange to a 

 crimson-red black. 



A few words on abnormal colouration must 

 conclude this portion of the subject. Possibly 

 the most familiar form of Heterochrosis is 

 Albinism. I dare say every reader can recall a 

 partial or complete instance of such a form of 

 abnormal colouration, say in the Blackbird or 

 the Pheasant, to quote the most familiar species. 

 This is due to the absence of black pigment. 

 Perhaps the next most familiar form of excep- 

 tional colouration is that which is technically 

 known as Melanism. This is exactly the reverse 

 of albinism, and is caused by an excess of black 

 pigment which renders the feathers abnormally 



