Orthoplasy with Correlated Variation 203 



of the sole becoming of a light colour which approximates 

 the light colour of the water when seen from beneath, and 

 the upper side a dull gray or mud colour, approximating the 

 surroundings when seen from above. 1 It is probable, there- 

 fore, that all these striking adaptations serve the great and 

 prime utility of concealment. If we now revert to the 

 accommodation of the position of the eyes, we find what 

 may be a striking illustration of the operation of organic 

 selection in screening correlated characters. 



For we may assume that so long as the adaptations in 

 coloration, and especially in the position of the eyes, were 

 not secured, or were only partially evolved, it would not 

 be of utility for the sole to lie on the side, for the upper 

 side would be exposed to view, and there would be the 

 disability arising from rendering one eye useless, if the 

 fish took such a position. But the coloration, in its 

 turn, could not be acquired through natural selection so 

 long as the fish did not lie on the side. Accordingly it 

 seems a fair inference that this whole group of adaptations 

 required such a gradual adjustment of the eyes ; that the 

 maintenance of the function of vision unimpaired was 

 absolutely necessary to the sole if he was to escape elimina- 

 tion, while gradually, as the adjustments of the position of 

 the eyes went on, he acquired variations placing him more 

 and more on one side, and also variations in the direction 

 of the requisite protective colouring. The straining of the 



1 Professor Osborn, who has kindly looked over the proofs of this chapter, 

 suggests (citing Cunningham's experiments in evoking colour on the lower 

 side of the sole, by throwing light upon it with a mirror ; cf. Cunningham on 

 'Recapitulation,' in Science Progress, I., N.S., pp. 483 ff.) that the colour-dif- 

 ferences are ontogenic, that is, that they are accommodations acquired in each 

 generation. This fully accords, in so far, with the position taken below, that 

 this adaptation is secondary to that of vision while the fish is in the flat position. 



