THE PHYSICAL CAUSE OF ANIMAL COLOUES 11 



It is doubtful how far the colours of animals are 

 caused by this principle ; but Dr. Gadow has given 

 strong reasons for supposing that the metallic 

 colours of birds' feathers are produced in this way,^ 

 and there are scales on the wings-cases of certain 

 beetles (Pachyrhynchus) which also may owe their 

 colours to refraction. 



All these causes of animal colours may be conve- 

 niently grouped under two heads — (1) pigmentary, and 

 (2) structural. The first head includes colours caused 

 by absorption, and the effects produced vary with the 

 chemical nature of the substance (pigment). The 

 second head includes the colours or appearances pro- 

 duced in all other ways, the efficient cause being the 

 structure of the substance rather than its chemical 

 nature. 



1 Froc Zool. Soc. 1882, pp. 409 et ae^. 



