284 ANIMAL DEFENCES 



organisms thus derive an extra benefit from the lighter colour 

 of the under side, as, of course, they are liable to be attacked 

 from below, which is not usually the case with terrestrial forms. 

 Besides fish, other interesting instances of this device may be 

 taken from the larger pelagic forms which drift or swim at or 

 near the surface. These, Wallace states (in Darwinism) ". . . 

 are beautifully tinged with blue above, thus harmonizing with 

 the colour of the sea as seen by hovering birds; while they are 

 white below, and are thus invisible against the wave-foam and 

 clouds as seen by enemies beneath the surface. Such are the 

 tints of the beautiful nudibranchiate mollusc Glaiicus atlanticus, 

 and many others." 



FLAT-FISHES. It has just been shown that compact, curved 

 forms are often made to appear flattish by a particular distribu- 

 tion of colour and shading, but it is clear that similar advantages 

 might be secured in an entirely different manner, i.e. by the body 

 actually becoming flat. A striking case of this is that afforded 

 by the Flat- Fishes, such as Sole, Turbot, Plaice, and the rest. 

 These, when in the condition of young fry, swim about like 

 ordinary fishes, and possess the same bilateral symmetry. Very 

 soon, however, they become laterally flattened, and take to living 

 on the sea-floor, with either the left or right side downwards, 

 according to the species. This side remains white or pale, though 

 not for the reason given in the case of animals with pale ventral 

 surfaces, and its eye migrates to the side which is kept upwards ; 

 otherwise it would be of no use. And, further, the upwardly- 

 directed surface becomes darkly pigmented so as to harmonize 

 with the sand or mud upon which the animal lives, the resem- 

 blance often being enhanced, much as in desert animals, by the 

 presence of spots and blotches of darker or different tint. Here, 

 however, as in so many other cases, protection is only afforded 

 by the coloration and marking when the animal remains at rest. 

 As in the case of the model already described (p. 283), movement 

 at once destroys the illusion, and in the case of a flat-fish the 

 attention of other animals must be attracted by the display, to 

 a greater or less extent, of the white or light-coloured side. The 

 fish would be much better protected if both sides were dark, but 

 the development of pigment in the skin is bound up with the 

 action of light, which is largely excluded from that side of the 

 body which faces habitually downwards. 



