210 METAMORPHOSIS AND 



change in eyes, fin, and colour to the adult Flat-fish 

 as we see it. All the evidence accumulated by the 

 experiments and observations of mutationists and 

 Mendelians goes to prove that this change is of an 

 entirely different kind from those variations which 

 are described as mutations, or as loss or addition of 

 genetic factors. 



This being the case, we have to inquire what is the 

 explanation of the evolution of the normal meta- 

 morphosis. 



The important fact is that the original symmetrical 

 structure of the larva and the asymmetrical structure 

 of the adult Flat-fish correspond to the different 

 positions of the body of the fish in relation to the 

 vertical, the horizontal ground at the bottom of the 

 water, and incidence of light. The larva swims 

 with its plane of symmetry vertical like most other 

 fishes ; its locomotion requires symmetrical de- 

 velopment of muscles and fins ; the two sides being 

 equally exposed to light, it requires an eye on each 

 side, and the pigment on each side is also related to 

 the equal exposure to light. The adult lying with 

 one side on the ground has its original plane of 

 symmetry horizontal and parallel to the ground, and 

 only the other side exposed to light, and on this side 

 only eyes and colour, i.e. pigment. The change of 

 structure corresponds with the change of habit. It 

 consists in the change of position of the lower eye, 

 the extension of the dorsal fin forwards, and the 

 disappearance of pigment from the lower side. In 

 the actual metamorphosis these changes take place 

 as the skeleton develops, before the hard bones are 

 fully formed, while the fish is still small, but the 

 young Turbot reaches a much larger size before 



