IN THE PLAICE. 95 



them directly, so that variations in the intensity of the light, 

 warmth, &c., could not produce contraction, or, on the other 

 hand, expansion, unless they were under the direct influence 

 of the rays. Some observations certainly have been made, 

 particularly those of Wittich, which prove that in animals 

 having the chromatic function (as the frog) the direct effect 

 from Ught-rays is, in fact, perceptible in a small degree ; but it 

 is now definitively established that this is not generally the 

 case, and that the changes of colour thus produced cannot be 

 included under the term ' chromatic function,' since no adap- 

 tation of colour to the surroundings is effected by them. Lister 

 demonstrated, on the contrary, by his experiments on frogs, as 

 long ago as 1858, that the activity of the chromatophores 

 in raises of chromatic function depends solely on the healthy 

 condition of the eye. So long as the eyes are in connection 

 with the brain by means of the optic nerves, the light reflected 

 from surrounding objects has a marked effect on the chro- 

 matophores ; but, so soon as the eyes are destroyed, or the optic 

 nerves are divided, the chromatophores also become totally in- 

 capable of perceiving any variations in the intensity of light 

 and colour ; thus the light reflected from objects can only affect 

 the colour of the skin by the interposition of the eyes. 



These observations were subsequently repeated by Pouchet, 

 who evidently was not aware of the preceding experiments, on 

 Fishes and Grabs ; and he, like Lister, came to the conclusion that 

 the irritation which excited the action of the chromatophores took 

 effect only through the eyes and optic nerves, and not directly 

 on the pigment-cells. Among the numerous new instances 

 which he brought forward, some of them highly instructive, 

 the case of a plaice observed by him is particularly interesting. 

 These fish have, as is well known, a white side which constitutes 

 the under surface, and a parti-coloured side which lies upper- 

 most ; this upper side exhibits the ' chromatic function ' in a 

 very high degi'ee. Among a great number of normal speci- 

 mens of the species which, on a white sandy bottom, were 

 alsQ whitish or very pale-coloured, he met with one single 

 dark-coloured fish in which, of course, the chromatophores 

 must have been in a state of relaxation, and this specimen was 



