,35,. THE EVOLUTION OF FLAT-FISHES. 195 



in the position of the eyes in the flat-fishes affects in any way the 

 sympathetic nerve of the lower side. 



Agassiz remarks in a foot-note that Pouchet has succeeded in 

 producing a white side in trouts by destroying the eye of that side. 

 As authority for this, he refers to an article in the Revue Scientifique 

 for 1877. I have not seen this particular article of Pouchet's, but I 

 have before me the latter's own abstract of his experiments on 

 the trout as described in two papers published by the Societe de 

 Biologie in 1876 and 1878. He states in this abstract that by taking a 

 trout from a river where the bottom was in some places light sand 

 and in others dark weed, and destroying one eye, he was able to 

 provoke a unilateral change of colour. The side opposite to that from 

 which the eye ivas vemoved became permanently dark, the chromato- 

 phores being paralysed and expanded. He says that the crossing of 

 the optic nerves being complete, one can conclude from this experi- 

 ment that the great sympathetic of each side depends directly on 

 the corresponding half of the brain. But this result was never 

 obtained in flat-fishes. The difference between Pouchet's investiga- 

 tions and Agassiz's description of them could not possibly be greater. 

 Pouchet does not even refer to the question of the origin or the 

 atrophy of the chromatophores ; Agassiz supports, as though it were 

 Pouchet's, a theory that the stimulation of the chromatophores 

 by reflex action through the eyes might not merely produce pigment, 

 but pigment of colours similar to the colours of the light acting upon 

 the eyes — that is, to the colours of the surroundings of the fish — and 

 thus explain the origin of what he calls mimicry — in more modern 

 language, protective resemblance. He puts aside the idea that 

 the absence of chromatophores or of pigment from the lower side 

 of flat-fishes is in any degree due to their withdrawal from the direct 

 action of light. He states that he placed young flounders in a glass 

 dish at a height over a table so that light could reach them from 

 below, and that this arrangement neither prevented the transfer of 

 the eye nor produced any effect in retaining the pigment spots of the 

 blind side longer than in specimens struck by the light only normally 

 from above. 



I have made a series of careful experiments of this kind with 

 young flounders taken at the beginning or in the middle of their 

 metamorphosis, improving on the method followed by Agassiz, by 

 placing a mirror below the aquarium at an angle of 45°, and cutting 

 off the light from above by means of an opaque cover. In the 

 Zoologischev Anzeigev of 1891 I published the result of the first of these 

 experiments, namely, that in the great majority of the specimens 

 treated in this way, after several months, although no effect was 

 produced upon the eyes, more or less of the skin of the lower side 

 was pigmented. 



This experiment of mine has attracted the attention of Professor 

 Alfred Giard, who criticises it in a recent note published in the 



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