58 PIGMENTS ON THE LOWER SIDES OF FLA.T FISHES. 



the lower sides. It cannot well be suggested, in explanation of this, 

 that light is not the cause of the pigmentation, but simply the absence 

 of mechanical contact over the parts of the skin which did not touch 

 the bottom in the slate tank. For, in the first place, the same reason 

 would apply to the fish in the glass-bottom tank, where there was no 

 sand, and the marginal parts of the body were equally separated from 

 the bottom ; and, in the second place, in the experiments with the 

 glass-bottom tank and mirror, the pigment when developed was most 

 abundant, precisely in those more central regions of the body which 

 were in contact with the glass bottom. I can only suggest at present, 

 that light reHected from rather dark coloured surroundings, is more 

 eflicient in producing the pigment than that coming directly from a 

 window and reflected by a silvered mirror. Mr. Poulton has shown 

 how remarkably the colour of lepidopterous pupa? is affected by the 

 colour of the surroundings to which the caterpillars arc exposed 

 during pupation. 



II. An Expekimem' on a Piebalu Plaice. 



In the previous number (p. 271) I described a piebald specimen of 

 the plaice, and suggested the possibility that if the lower side were 

 exposed to light the unpigmented area on the upper side would, imri 

 jmssu, become pigmented. This result was the logical consequence of 

 the hypothesis I put forward in explanation of the fact that, in such 

 specimens, part of the upper side remains unpigmented, while con- 

 tinuously exposed to light in the same way as normal specimens. 

 After writing the contribution to which I refer, I instituted the experiment 

 with the same specimen, not, I must confess, with very sanguine hopes 

 of its success. The specimen had lived in the aquarium from October 

 3rd to the month of December, without showing any change in 

 pigmentation. I then placed it in a large bell-jar, without any sand, 

 gravel, or other objects in the water, and placed the jar over a mirror 

 in front of a south window. The top and front of the jar w^ere shaded 

 with black paper, so that the upper side received little light, the lower 

 side a great deal. I left the Laboratory on February 2nd, in order to 

 take charge of the east coast work at Grimsby, and Mr. Allen, the 

 new Director of the Laboratory, kindly consented to take this and 

 other experiments of mine under his care. I am most grateful to 

 him for the fidelity with which he executed this trust. When I 

 returned to Plymouth, on June 17th, I found the plaice still alive 

 and in good health. The fish, on examination, was found to be 7 inches 

 long. On the lower side, where previously there was no trace of 

 pigment, there were numerous small patches, scattered principally, 



