COLOUHS OF ANIMALS. 



171 



environment of tlie animal, cannot be produced. Some- 

 what similar changes of colour occur in some prawns 

 and flat-fish, according to the colour of the bottom 

 on which they rest. This is very striking in the 

 chameleon shrimp (My sis Chamceleon), which is grey 

 when on sand, but brown or green when among 

 sea-weed of these two colours. Experiment shows, 

 however, that when blinded the change does not occur ; 

 so that here too we probably have a voluntary or 

 reflex sense-action. 



These peculiar powers of change of colour and adap- 

 tation are, however, rare and quite exceptional. As a 

 rule, there is no direct connection between the colours of 

 organisms and the kind of light to which they are 

 usually exposed. This is well seen in most fishes and 

 in such marine animals as porpoises, whose backs are 

 always dark, although this part is exposed to the blue 

 and white light of the sky and clouds, while their bellies 

 are very generally white, although these are constantly 

 subjected to the deep blue or dusky green light from the 

 bottom. It is evident, however, that these two tints 

 have been acquired for concealment and protection. 

 Looking down on the darlc back of a fish it is almost 

 invisible, while, to an enemy looking up from below, the 

 light under-surface would be equally invisible against 

 the light of the clouds and sky. Again, the gorgeous 

 colours of the butterflies which inhabit the depths of 

 tropical forests bear no relation to the kind of light 

 that falls upon them, coming as it does almost wholly 

 from green foliage, dark brown soil, or blue sky ; and 

 the bright under wings of many moths, which are only 

 exposed at night, contrast remarkably with the sombre 



