374 FISHES 



In these two features we find obvious functional advantages 

 in relation to the conditions of life, and they are thus in harmony 

 with the explanation of adaptations by the theory of natural 

 selection, or survival of the best adapted. But in the third 

 feature no such advantage is proved. The colour on the upper 

 side is doubtless some advantage to the fish in masking it from 

 enemies or from prey, and the benefit is much increased by 

 the faculty which the fish possesses of changing the intensity 

 of the colour to bring it into harmony with that of the ground 

 on which it rests. The absence of pigment on the lower side, 

 on the other hand, has never been proved to be of any benefit 

 to the fish. The suggestion that this colour harmonises with the 

 light coming from above when the animal is seen from a lower 

 level, and is therefore useful in concealing the fish, is open to 

 two objections. Firstly, the fish is not attacked from below, 

 and always seeks protection by covering itself with sand, and 

 secondly, when the fish is observed from below it appears as an 

 opaque object shutting off the light, and is therefore conspicuous. 

 There is very strong evidence that the reason why the lower 

 side has lost its pigment is that it is shaded from the light. We 

 do not know exactly how the action of light produces pigment, 

 but in fishes at least we have good reason to believe that this 

 action is necessary for the production of pigment in the skin. 

 Fishes, like the cave-fishes Amblyopsis and Lucifuga, which live 

 in the dark, are colourless, and fishes generally are colourless on 

 the ventral side. The present writer, moreover, has shown that 

 if flounders are kept alive in an apparatus which exposes their 

 lower sides to light, pigment is gradually developed on the skin 

 of the lower sides. The experiments were carried out at the 

 Laboratory of the Marine Biological Association at Plymouth 

 during the years 1890- 1 893. The apparatus consisted of a 

 shallow tank the bottom of which was made of plate glass, on 

 to which light was directed upwards by means of an inclined 

 mirror. Young flounders soon after their metamorphosis were 

 placed in this tank, and after a period varying considerably in 

 different specimens, pigment precisely similar to that of the 

 upper side began to show itself on the lower side. In some 

 specimens kept under these conditions from one to three years 

 the lower sides became almost as completely pigmented as the 

 upper. Fig. C. on Plate XXX. represents one of these speci- 



