2 ANIMAL COLOEATION. 



surfaces. The metallic lustre of the feathers of many birds, 

 such as the humming birds, is due to the presence of 

 excessively fine strias upon the surface of the feathers. Dr. 

 Gadow has recently gone into the question of the colours of 

 birds' feathers as determined by their structure in great detail,* 

 and to his paper the reader is referred ; in every case the 

 colour needs for its display a background of dark pigment ; 

 that this is so is shown very well in the case of the albino 

 forms of many birds: the structure of the feathers is perfectly 

 normal, but the pigment needed as a background to show up 

 the effect of the feather structure is wanting. 



Colours caused by Absorption of Light due to the Presence 



of Pigments. 



By far the commonest source of colour in invertebrate 

 animals is the presence in the skin of definite pigments which 

 absorb all the rays of light except those of a particular wave- 

 length, thus giving the effect of a particular colour. Pigments 

 ai-e found also in the hair and skin of mammals, in the feathers 

 of birds, and in the horny integument of reptiles. A great 

 variety of pigments have been approximately isolated and their 

 chemical nature studied ; but it would be beyond the scope of 

 the present work to attempt any general account of animal 

 pigments. Those interested in the subject may refer to Kru- 

 kenberg's " VergleichendphysiologischeStudien," to papers by 

 Dr. Sorby, Dr. Macmiinn and others in the Proceedings of the 

 Royal Society, the Quarterly Journal of Microscopical Science 

 and elsewhere. 



There are a few facts, however, which may be noticed here. 



The same colour even in allied forms is not always due to 

 the presence of an identical pigment. Thus the brown colour of 



* Proc. Zool. Soc. 1882. 



