216 METAMORPHOSIS AND 



of the lower eye to the upper side and of the forward 

 extension of the dorsal fin has not been obtained, 

 though years ago I made some attempts, at the 

 suggestion of Mr. G. J. Romanes, to obtain such 

 evidence with regard to the eye by keeping young 

 Flounders, already partially metamorphosed, in a 

 reversed position. I did not succeed in devising 

 apparatus which would keep the young fish alive in 

 the reversed position for a sufficiently long time. 

 We can only consider, therefore, whether those other 

 changes can reasonably be attributed to the con- 

 ditions of life. Anatomical investigation shows that 

 the bony interorbital septum composed principally 

 of the frontal bones, which in symmetrical fish passes 

 between the eyes, is still between the eyes in the Flat- 

 fish, but has been bent round through an angle of 

 90 degrees on the upper side, while in the lower side 

 a new bony connexion has been formed on the outer 

 side of the eye which has moved from the lower side. 

 This connexion is due to a growth from the pre- 

 frontal backwards to join a process of the frontal, and 

 is entirely absent in symmetrical fishes. It is along 

 this bony bridge that the dorsal fin extends. The 

 origin of the eye muscles and of the optic nerves is 

 morphologically the same as in symmetrical fishes. 

 On the theory of modification by external stimuli we 

 must naturally attribute the dislocation of the eye of 

 the lower side to the muscular effort of the fish to 

 direct this eye to the dorsal edge, but something 

 may also be due to the pressure of the flat ground on 

 the eye-ball. There is little difficulty in attributing 

 the bending of the interorbital septum to pressure 

 of the lower eye-ball against it, pressure which is 

 probably due partly if not chiefly to the action of the 



