68 THE COLOUES OF ANIMALS 



appendages, most of which are supported by a bony core. 

 The appendages are flat, and are alternately banded 

 with dusky brown and orange, exactly resembHng the 

 form and colour of the sea-weed to which the fish 

 clings with its tail. There also are many bony spines 

 without the flat folds of skin, and these are doubtless 

 defensive. 



The general arrangement of colour on porpoises, 

 most fish, &c., has been well explained by Wallace. 

 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 Hght of the clouds and sky.'^ 



The white colour of one side of such fish as the 

 sole, turbot, &c. {Pleuroneetidce)^ viz. the side which 

 is in contact with the sand or mud, cannot be ex- 

 plained in this way. In such a case we see the dis- 

 appearance of colour in consequence of the cessation 

 of natural selection, as in the white eggs laid in the 

 dark, while the white bellies of many fish may be 

 compared to the whiteness of the eggs of the wood- 

 pigeon, an appearance produced by the operation of 

 natural selection. 



It has been already pointed out that natural 

 selection may not only remove the pigment from an 

 animal, but may even replace the red blood of a 

 vertebrate by a colourless fluid. The transparency 



^ Tropical Nature, p. 171. 



