86 Development or Flat-fishes 



halibuts, and the long rough dab swim on their left 

 side; turbot, brill, and megrims on their right side. 

 The down-turned side is always the heavier. Its sur- 

 face is silvery white, and that implies that there is 

 no pigment, the whiteness being due to crystalline 

 spangles of a waste product called guanin. On the 

 floor of an aquarium in which young flounders were 

 developing, Mr. J. T. Cunningham placed a mirror 

 throwing the light upwards, and the result was 

 that eleven out of thirteen of the metamorphosing 

 flounders developed pigment on the down-turned, as 

 well as on the up-turned, side. Another peculiarity of 

 the flat-fishes is that both eyes are on the up-turned 

 side — an obvious advantage since an eye on the down- 

 turned side would be not only useless but a source of 

 danger, as it would be very liable to be scratched. 

 The eye on the down-turned side travels round or in 

 part through the margin of the head. In these and in 

 other ways the bony flat-fishes are very peculiar ; but 

 there is no alternative to the view that they originated 

 from an ordinary symmetrical stock. This is prac- 

 tically proved by the life-history, for the newly 

 hatched forms are quite symmetrical and swim near 

 the surface in the ordinary position of other fishes. 

 As they go on developing a marked lopsidedness sets 

 in ; they seem to lose their poise ; they begin to sink 

 towards the floor of the sea. We cannot make sense of 

 the story except on the evolutionist interpretation 

 that the young flat-fishes are recapitulating the his- 

 tory of their race. 



