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Colouration of 3flat Jfiebea, 



WHOEVER has seen flat fishes alive, or even dead, but not 

 divested of their skin, must have remarked the notable 

 difference existing between the colour of the dorsal 

 surface, exposed to the water, and the ventral surface which in the 

 livine state faces the bottom. While the dorsal surface is more or 

 less covered, the ventral surface remains white. What is the 

 meaning of this ? The Weismann School — rather more Darwin- 

 ian than Darwin himself — insists upon attributing the fact to 

 natural selection. And this school, according to which the environ- 

 ment acts upon the living being, ascribes it to a physical influence 

 — to the fact that the ventral surface naturally receives much less 

 light than the dorsal. In truth, one can scarcely see what natural 

 selection has to do with it. From the standpoint of the latter the 

 colouration of the ventral surface seems indifferent, and, if it is 

 not, it is permissible to think that it would be more advantageous 

 to the fish to have this surface grey, like the dorsal, than white ; 

 that is to say, conspicuous. 



Mr. Cunningham, of the Maritime Biological Association of 

 Plymouth, has recently studied the phenomenon, and does not 

 conceal his sympathy for the theory of the action of the environ- 

 ment. He experimented with young flounders ( Pleuronectus 

 flexus), whose eye had not left the ventral surface. The pigment 

 of the latter had already disappeared in great part. The animal 

 was already lying on this side, and on the dorsal the pigmentation 

 was pronounced. Mr. Cunningham made the following experi- 

 ment : — 



Darkening the cover and sides of a glass vessel, he placed the 

 latter, containing some young flounders, upon a support, and 

 beneath it arranged a mirror that reflected the solar light upon the 

 bottom, so that the dorsal surface was exposed to darkness and 

 the ventral to light ; he reversed the normal conditions. The 

 water was copiously renewed, and the fish had all the food that 

 they needed. Other fishes were placed in a small vessel, which 

 was normally lighted. The results were as follows : — Out of the 

 thirteen fishes that received light from beneath, only three remained 



