200 METAMORPHOSIS AND 



believe that Amphibia came out of the water and 

 breathed air because by an accidental mutation they 

 possessed lungs and a pulmonary circulation capable 

 of atmospheric respiration. Such is the result of 

 applying conclusions derived from phenomena of 

 one kind to phenomena of a totally different kind. 

 One of the chief differences between structural 

 features and correlations which are adaptive from 

 those which are not is the process of metamorphosis, 

 where we see the structure changing in the in- 

 dividual Hfe history as the mode of life changes. 

 The egg of the Flat-fish develops into a symmetrical 

 pelagic larva similar to that of many other marine 

 fishes. The larva has an eye on each side of its head 

 and swims with its plane of symmetry in a vertical 

 position : it has also colour on both sides equally. 

 When the skeleton begins to develop the transforma- 

 tion takes place : the eye of one side, left in some 

 species, right in others, moves gradually to the edge 

 of the head and then on to the other side. The 

 dorsal fin extends forward, preserving its original 

 direction, and so passes between the eye that has 

 changed its position and the lower side of the fish, 

 on which that eye was originally situated. In some 

 cases this extension of the fin takes place earlier and 

 the eye passes beneath the base of the fin to reach the 

 other side. Any one who takes the trouble to make 

 himself acquainted with the facts will see that the 

 three chief features of the Pleuronectid — namely, 

 the position of the eyes, the extension of the dorsal 

 and ventral fins, and the absence of pigment from 

 the lower side — are not structurally correlated with 

 one another at all as changes in different parts of the 

 organism in a mutation are said to be, but are all 



