PDF - Wallace Online

348 TROPICAL NATUREweed of these two colours. Experiment shows, however,that when blinded the change does not occur; so thathere too we probably have a voluntaryor reflex senseaction.These peculiar powers of change of colour and adaptationare, however, rare and quite exceptional. As a rule there isno direct connection between the colours of organisms andthe kind of light to which they are usually exposed. This iswell seen in most fishes, and in such marine animals as porpoises,whose backs are always dark, although this part isexposed to the blue and white light of the sky and clouds,while their bellies are very generally white, although theseare constantly subjected to the deep blue or dusky greenlight from the bottom. It is evident, however, that thesetwo 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 undersurfacewould be equally invisible against the lightof theclouds and sky. Again, the gorgeous colours of the butterflieswhich inhabit the depths of tropical forests bear norelation to the kind of light that fallsupon them, coming asit 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 remarkablywith the sombre tints of the upper wings, which aremore or less exposed to the various colours of surroundingnature.Classification of Organic ColoursWe find, then, that neither the general influence of solarlight and heat, nor the special action of variously tinted rays,are adequate causes for the wonderful variety, intensity, andcomplexity of the colours that everywhere meet us in theanimal and vegetable worlds. Let us therefore take a widerview of these colours, grouping them into classes determinedby what we know of their actual uses or special relations tothe habits of their possessors. This, which may be termedthe functional and biological classification of the colours ofliving organisms, seems to be best expressed by a divisioninto five groups, as follows :