Ill 



evident that the sole in this condition has, like the gravel, a variegated colouring 

 which at some distance from the eye renders it less conspicuous. The black spots and 

 the small white spots resemble the black and white pebbles of the gravel ; but on the 

 other hand, the continuous streak of opaque white along the edge of the fins is 

 distinctly conspicuous. A sole on gravel of this kind in a tank of some size is usually 

 either completely or partially buried under the gravel, and when the fins are thus 

 concealed and only part of the body exposed the sole may easily escape notice from a 

 human observer. On the other hand, it is also very easy to discover a sole in such a 

 condition when one looks for it. I believe that soles are seldom on gravel in their 

 natural state ; for I have found that on gravel or sharp sand they nearly always sooner 

 or later injure their fins or skin, and that abrasions so produced usually lead to 

 inflammation which causes death. 



I placed another sole in a large shallow dish of white porcelain of the kind used in 

 photographic manipulation. No material of any kind was placed in the dish, the fish 

 rested on the smooth white surface of the porcelain, and it was' exposed to the full 

 daylight of the south windows of the Laboratory. Plate III is a reproduction of the 

 water colour drawing made from the sole in this condition. The paleness of the 

 colouring is extraordinary. The darkest tint in the blotches is a straw colour, 

 scarcely darker than the yellow of the fin-membranes. The ground-colour is a pale 

 grey with a slight tinge of blue in places. The white spots have disappeared entirely, 

 a result which would not have been expected : the spots where they existed are 

 somewhat blue. The dark spot on the fin is the darkest colour in the whole surface, 

 and has still streaks of dark brown between the fin rays. The sole used in this 

 experiment was a male 10^ inches (26-7 cm.) long; but I founcj that the sole from 

 which Plate I was drawn became as pale as this when placed under the same 

 conditions. 



The drawing of the sole just described was finished on June 8, 1889. This sole 

 died shortly afterwards and its appearance the day after death is represented on PI. IV. 

 Another sole was taken for the drawing given in PI. I. I found when this latter 

 specimen was placed on the white porcelain that it became in a few minutes as light 

 as the figure on PI. Ill, but after it had been left all night in the same condition, on 

 the following morning the small spots which on gravel were opaque white had become 

 quite a dark grey and formed a marked contrast to the surrounding yellow. 



To ascertain the maximum darkening of colour possible I used the same sole from 

 which PI. I was taken. At first I found it difficult to produce any colouring much 

 darker than that of PI. I. I lined the porcelain dish with black paper and placed the 

 fish on that, but it showed much the same colours. Then I varnished the dish with 

 black varnish of a deeper black than the paper, but got no better result. It then 

 occurred to me that the maximum of darkness could not be obtained until the quantity 

 of light was reduced ; accordingly I placed some washed coal at the bottom of a tub 

 about nine inches deep, and placed the sole on this ; then I placed the tub on the 



