212 Darwin, and after Darwin. 



or loss of colour is in some cases brought about by 

 natural selection, on account of the original colour 

 being correlated with certain physiological characters 

 (such as liability to particular diseases, &c.) ; so that 

 when natural selection operates directly upon these 

 physiological characters, it thereby also operates 

 indirectly upon the correlated colours. But to suppose 

 that this can be the explanation of the uniform 

 diminution of colour in all inhabitants of dark caves 

 would be manifestly absurd. If there were only one 

 class of animals in these caves, such as Insects, it 

 might be possible to surmise that their change of 

 colour is due to natural selection acting directly upon 

 their physiological constitutions, and so indirectly 

 upon their colours. But it would be absurd to 

 suppose that such can be the explanation of the 

 facts, when these extend in so similar a manner over 

 so many scores of species belonging to such different 

 types of animal life. 



With more plausibility it might be held that the 

 universal discharge of colour in these cave-faunas 

 is due, not to the presence, but to the absence of 

 selection i.e. to the cessation of selection, or pan- 

 mixia. But against this at all events as a full or 

 general explanation lie the following facts. First, 

 in the case of Proteus which has often been kept 

 for the purposes of exhibition &c., in tanks the skin 

 becomes dark when the animal is removed from the 

 cave and kept in the light. Secondly, deep-sea faunas, 

 though as much exposed as the cave-faunas, to the 

 condition of darkness, are not by any means invariably 

 colourless. On the contrary, they frequently present 

 brilliant colouration. Thus it is evident that if pan- 



