ANIMALS AND THEIR ENVIRONMENTS. 307 



The first point in their history to which attention may be 

 directed is that in their early life and when the young fish 

 emerges from the egg, the eyes are situated where we should 

 naturally expect to find these organs one on each side of 

 the head. Moreover, in the days of its youth, the flat-fish is 

 thoroughly symmetrical in all other respects, even to the 

 coloration of its body, the two sides being tinted of the same 

 light hue. Soon, however, a change of structure and confor- 

 mation begins to be apparent, especially in the head-region. 

 The eye of the lower side on which the fish is destined to 

 rest begins literally to travel round to the upper side of the 

 body j this process taking place merely through a curious 

 malformation and twisting of the bones of the head, and not 

 by means of the eye passing through the skull, as was for- 

 merly supposed. Then also the colour of the upper side of 

 the body gradually deepens and acquires the tint of adult 

 life ; a hue admirably in harmony with the surrounding sand, 

 and rendering the detection of these fishes as they rest on 

 the sandy sea-bed a matter of extreme difficulty, as any one 

 who has " speared " flounders knows. The causes of the 

 development of colour on the upper surface may doubtless, as 

 Darwin remarks, be attributed to the action of light ; but it 

 is notable that in some flat-fishes there exists a chameleon- 

 like power of altering the tint of their bodies so as to bring 

 them into harmony with the particular colour of their sur- 

 roundings. The acquirement of this latter condition 

 becomes allied to that termed " Mimicry ; " but to explain 

 the development of the power of changing colour, we must 

 call to aid conditions other than that of the action of light, 

 and which affect and influence the more intricate and hidden 

 forces of living beings. Thus are gradually acquired the 

 peculiar features which mark the adult existence of these 

 fishes. The chronicle of their early life and history impresses 

 one fact primarily on our minds, namely, that if their 

 development is to be held as furnishing a clue to the origin 

 of their modifications, the knowledge that at first the flat- 

 fishes possess symmetrical bodies demonstrates that origin- 



