348 TROPICAL NATURE 



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 adaptation 

 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 por- 

 poises, 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 dark 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 butter- 

 flies 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 tints of the upper wings, which are 

 more or less exposed to the various colours of surrounding 

 nature. 



Classification of Organic Colours 



We find, then, that neither the general influence of solar 

 light and heat, nor the special action of variously tinted rays, 

 are adequate causes for the wonderful variety, intensity, and 

 complexity of the colours that everywhere meet us in the 

 animal and vegetable worlds. Let us therefore take a wider 

 view of these colours, grouping them into classes determined 

 by what we know of their actual uses or special relations to 

 the habits of their possessors. This, which may be termed 

 the functional and biological classification of the colours of 

 living organisms, seems to be best expressed by a division 

 into five groups, as follows : 



