334 Animal Life 
kept alive in large bell-jars, or in a wooden tank fitted with a glass bottom, and lght 
was reflected upwards on to the lower side of the fish by means of large mirrors. A 
photograph of the apparatus is reproduced in Fig. 2. The result was that after some 
months more or less pigment was developed on the lower side of the fish, and in some 
cases where the experiment was continued for a long time on the same fish, the 
lower side became almost as much pigmented as the upper. 
In the case of the specimens which occur in nature having either white areas on 
the upper side or coloured areas on the lower side, there is no evidence that these 
peculiarities are due to the action of light. We have no reason to believe that when a 
white patch occurs on the upper side, that patch has been covered or shaded in any way. 
A plaice could not very well pass the whole of its life beneath a stone with only its 
head and tail exposed, nor could it stay in one spot with only the middle of its body 
buried beneath the gravel or sand. There is in fact nothing to indicate that white 
patches on the upper side have been less exposed to light than the coloured parts of 
the skin. Conversely, it is not probable that coloured patches on the lower side have 
been exposed to the light more than the white parts of that side, or more than the 
whole side in normal cases. In the coloration produced in my experiments, the boundary 
of the coloured areas was not sharply defined as in the natural variations, but there was 
a gradual transition from the coloured region to the white. In some cases in nature a 
certain amount of light may reach the lower side, especially in the case of flounders 
which live on hard ground in shallow water, and I believe pigment is sometimes produced 
on the lower sides in this way. But this does not apply in the case of sharply-defined 
coloured areas, which must be regarded as spontaneous natural variations. 
Thus there is an apparent contradiction between these natural variations and the 
results of my experiments. In the latter, exposure to light caused the formation of 
pigment in the skin, and yet a white patch on the upper side in the piebald plaice is 
continually exposed to light without any pigment being produced. I have maintained 
that the absence of pigment in normal cases is due to the absence of light, and yet 
specimens occur in which the lower side is coloured like the upper, and these fish lhe 
on the ground like other flat-fishes and the skin is pigmented in spite of the absence 
of light. Double or ambicolorate specimens of this kind are not uncommon in the turbot, 
and the condition when fully developed is associated with a malformation or abnormality 
of the dorsal fin, the base of which is detached from the head at the anterior extremity 
of the fin, so that the end of the fin projects forwards in a kind of hook. How are 
these apparent contradictions to be explained? How can the results of the experiments, 
which seem to support the view that the absence of colour from the lower sides of 
flat-fishes is due to the absence of light, be reconciled with the occurrence of these 
spontaneous variations ? 
The occurrence in nature of pigmentation on the lower side of a flat-fish is often 
regarded as an instance of reversion. The ancestral flat-fish was doubtless equally 
coloured on both sides, like an upright fish such as the John Dory, and when the 
{lat-fish is equally coloured on both sides, or partially coloured on the lower side, it is 
natural to conclude that it has reverted more or less completely to the ancestral 
condition. But on the other hand, when the upper side is partially white, this cannot 
be due to reversion, for it is impossible that both of the variations in opposite directions 
can resemble the ancestral fish. Therefore it is by no means certain that the occur- 
rence of pigment on the lower side is due to reversion, and in fact both kinds of 
variation may be regarded simply as variations in opposite directions. 
Now these variations in both cases are variations of an asymmetrical animal towards 
symmetry—that is to say, either side may in individual cases more or less completely 
imitate the other, instead of differmg from it. Such variations are known to occur 
