THE STRUCTURE OF BL&l) FISHES. 51 



Natural selection can not have affected the coloration of the 



cave forms, for it can be absolutely of no consequence whether a 



cave species is white or black. It could affect the coloration only 



•indirectly in one of two ways: First, as a matter of economy, but 



since the individual is in part bleached by the direct effect of the 



darkness there is no reason why natural selection should come into 

 play at all in reducing the pigment as a matter of economy; second, 

 Romanes has supposed that the color decreases through the selec- 

 tion of correlated structures — a supposition he found scarcely con- 

 ceivable when the variety of animals showing the bleached condi- 

 tion is considered. 



