76 SUMMER ISLANDS AND A SUMMER SEA 



changes — hardly surpassed by the chameleon — which 

 some of these file-fishes undergo. They are particularly 

 conspicuous in Batistes macutatuSy which, when swimming 

 in clear water, is blue- black with numerous light-blue 

 spots, but which when concealed in dark, discoloured 

 drift-wood, can occlude its spots and appear as black 

 as the wood itself. A species of Monacanthus (I think 

 M, scriptus) is perhaps even more chameleonic : one 

 that I caught on a piece of drift was of a dark greenish- 

 black colour, scribbled over with fine yellow lines, but 

 when turned into a white bucket of seawater, the 

 yellow lines gradually expanded until the fish became 

 pale yellow, minutely and sparsely speckled with black. 

 The mechanism of these colour-changes is well known ; 

 they depend on the expansion and contraction of 

 certain large skin-cells in which the pigments are 

 imbedded, . and the stimulus from the brain which 

 causes the change in size of these cells is awakened 

 by a message from the retina and optic nerve, the 

 whole train of events being an unconscious reflex 

 action much like breathing and swallowing. Their 

 utility is also well understood : briefly, they enable an 

 animal to conceal itself without any sacrifice of its 

 independence or any curtailment of its liberty. If on 

 first thoughts it seems rather unreasonable to suppose 

 that it would in any way benefit a fish to change its 

 colour to that of a drift tree, it must be remembered, 

 first, that we are here only speaking of fishes that 



