LIGHT AND COLOUR 151 



treated were found to have developed spots of colour 

 on their under-surface, and thus to present an ambi- 

 colourate appearance. Ultimately one of the fish 

 became almost totally embrowned, and lost all but 

 mere traces of its usual silvery whiteness. 



More convincing even than this experiment is the 

 one which Nature herself has carried out on the sucker- 

 fish. The sucker-fish is a parasite hanging on to the 

 bodies of larger tropical fish for the sake of the crumbs 

 that fall at meal-times. To ensure its hold the fish 

 has converted its dorsal fin into a strong, transversely 

 ridged sucker, which it applies to the upper side of its 

 host, and thus views the world standing on its head. 



Its back is in shade, its belly in light, with the 

 result that the usual colouration of animals is here 

 reversed — its back is white and the belly dark, almost 

 the only exception to the otherwise universal rule ; and 

 so accustomed are we to it, that when we first handle 

 a sucker-fish we involuntarily turn it upside down and 

 unconsciously acknowledge the strength of that asso- 

 ciation that leads us to regard the dark surface of an 

 animal as the upper one. 



Whilst we can thus conclude that the shaded parts 

 of the body become pale and the exposed parts dark, 

 there are some natural experiments upon the total 

 bleaching of the body by generations of cave life. 

 The cave newts of Carniola, in Hungary (fig. 28), have 

 become famous. They are milky white blind newts, 

 a foot or so in length, and live in the darkness and 

 cold of subterranean waters. Unlike their allies, the 



