118 COLORATION OF THE SKINS OF FLAT-FISHES. 
these chromatophores come? ~ Do they migrate along the skin or 
through the tissues of the body from the skin of the upper side? or 
are they formed in situ, and, if so, how? We are not prepared at 
present to answer these questions definitely. We believe, however, 
that their presence is not due to migration, but that the pigment is 
formed from the elements supplied by the blood at or immediately 
near the place where they appear. That they do not come round 
the edge of the body along the tissues of the skin is proved by the 
fact that spots and patches of pigment may appear in any position, 
and quite isolated, on the lower side. 
It is important to mention that, although chromatophores are 
present on both sides of the fish in the intermediate stages of meta- 
morphosis at which most of these experiments have commenced, the 
action of light on the lower side never results in the retention of 
these chromatophores. The latter disappear from the lower side 
completely, and after prolonged action of the light they reappear. 
It is certain, therefore, that the disappearance of the pigment from 
the lower side in the normal flat-fish is an hereditary character, and 
not due to the withdrawal of the action of lght in the individual. 
If the latter were the case, of course the pigment would be retained 
permanently from the larval stage as soon as the hght was allowed 
permanently to act upon the lower side of the fish. The disappear- 
ance of the pigment is, therefore, an hereditary family character in 
the Pleuronectidee. On the other hand, the fact that in these ex- 
periments the pigment, after prolonged action of the light, actually 
reappears is strong evidence (to my own mind a proof) that originally, 
in the beginning of the evolution, the pigment disappeared in conse- 
quence of the withdrawal of the lower sides from the action of light. 
If this be granted, it follows, of course, that a character originally 
acquired has become hereditary. 
Pigment occurs as an occasional variation on the lower sides of 
flounders living free under natural conditions. That this does not 
invalidate the significance of these experiments is shown by the fact 
that in a number reared under normal conditions in the aquarium, 
only 1 in 90 showed a spot of pigment on the lower side, while of 
94 specimens obtained from the estuary of Hamoaze only one 
showed a coloured spot on the lower side. Moreover, in the experi- 
ments the extent of the pigmentation, and the number of specimens 
exhibiting it, steadily increase from month to month, while in nature 
pigment on the lower sides is not any more common in large specimens 
than in small. 
The above is a mere brief summary of general results and conclu- 
sions. The full description of the investigations, with illustrations, 
is contained only in the memoir communicated to the Royal Society. 
