vii] HEREDITY 91 



live in the dark the pigment in the skin is frequently 

 absent, as it is also in flat-fish on the side of the body 

 which lies protected from light on the sea-floor. It is 

 said that pigment in such cases cannot be harmful, and 

 so its disappearance is not due to natural selection. 

 But pigment very generally appears in response to 

 the action of light, and so it is supposed that the 

 absence of the stimulus to production, acting for 

 many generations, has caused the pigment to dis- 

 appear. This is illustrated by the well-known 

 experiment of Cunningham on flat-fish. The young- 

 fish is pigmented on both sides of the body ; it 

 then settles on one side and the pigment on that 

 side disappears. Cunningham reared such young 

 fish in an aquarium lighted from below : when they 

 settled on the bottom the pigment disappeared, but 

 if kept still longer exposed to light from beneath, 

 the pigment began to come back again. The dis- 

 appearance of the pigment, although exposed to 

 light, proves that the loss is hereditary ; its return 

 on continued exposure to light is interpreted by 

 Cunningham to mean that its disappearance was 

 due to absence of light, and has gradually become 

 hereditary, but that the process can be reversed by 

 again exposing the lower side to the action of the 

 stimulus. The same argument has been used with 

 regard to the colourless skins and vestigial eyes of 

 animals living in caves ; where the structure is use- 



