A Piebald Plaice 335 
in all animals in which the two sides are normally different. From this point of 
view the white patch on the upper side of our piebald plaice may be regarded as 
imitating the lower. Then if the lower side is white in consequence of absence of 
light, the white patch will imitate it although itself exposed to light. So also may 
be explained the fact that, when’ pigment is present on a part of the lower side, it 
persists in spite of the fact that there is no lght to affect it. It is there because 
part of the lower side is imitating the upper, is developing in the same direction as 
the upper, mstead of in its own proper direction. 
We may then, as it were, regard the white patch as belonging physiologically to 
the lower side, and these curious questions arise: If the lower side in this specimen 
were exposed continually to the light, would it develop pigment as in the experiments 
with ordinary flounders ? 
And secondly, what would 
happen to the white patch 
on the upper side? Would 
that develop pigment too? 
This experiment was 
actually tried. The speci- 
men lived in the aquarium 
from October 3rd to the 
month of December without 
showing any change in the 
lower side or in the white 
patch on the upper. It was 
then put into a large glass 
bell-jar, under which was a ; 
mirror, opposite a window. 
On June 17th following the 
fish was carefully examined. 
Tt was then seven inches | 
: Photo by J. T. Cunningham. 
long. On the lower side, Fig. 2. Apparatus for exposing the lower sides of flat-fishes to light 
where previously no pig- reflected from mirrors. 
ment could be seen, there 
were numerous small patches of colour, as seen in Fig. 1, which is a copy of a 
drawing made from the living fish. Exactly similar coloured spots had also appeared 
in the white patch on the upper side, as shown in the upper figure of the coloured plate. 
This experiment, therefore, supports very strongly the view explained above of the 
meaning of these variations. The variations themselves, whether consisting in the presence 
of pigment on the lower side or the absence of pigment on the upper, are not due to 
special conditions, but are spontaneous. But when white occurs on the upper side it is 
not a case of albinism, like the white in a piebald horse or mouse, but is connected 
with and dependent upon the absence of pigment on the lower side; and when the 
lower side is exposed to light, pigment appears both on that side and in the white area 
of the upper side. 
