438 ME. B. W. h HOLT — STITDIE8 IN [May 1, 



We have therefore, in the ambicolorate but otherwise normal 

 Brill, a reproduction on the blind side of those markings which are 

 most conspicuous in the young of the same and of most other 

 species of Pleuronectids, and which are characteristic, of course 

 on the ocular side, of the smallest species of the genus Rhombus 

 and its immediate allies. I wish to draw especial attention to 

 these facts, as I consider that they have a distinct bearing on the 

 interpretation of the phenomenon of ambicoloration. 



Mr. Cunningham, in the Royal Society memoir so frequently 

 referred to, establishes the fact that pigment can be produced by the 

 action of light on the colourless under surface of an already meta- 

 morphosed flat-fish, noting, at the same time, the great variation 

 which exists in the susceptibility of individuals to this treatment. 

 The authors reject, however, the hypothesis that the ambicolorate 

 condition so commonly met with can also be explained as due to 

 the action of light, since there is " not the slightest evidence at 

 present that these abnormal specimens have been exposed to 

 abnormal conditions, or have had abnormal habits of life," and, for 

 my own part, I most certainly agree with them in the main. There 

 is, however, one colour abnormality, so common as perhaps hardly 

 to merit such a designation, which I think is probably due to the 

 action of light. This is the presence of more or less pigment on 

 the blind side of the median fins. It may be present merely in 

 the form of irregular dull streaks or splashes, as in the caudal fin 

 of the Sole, which forms the subject of this paper, or it may be 

 developed to such an extent that practically the whole of the 

 marginal fins are coloured. This last condition is exemplified by a 

 Dab (PL Umancla) in my possession, but the pigment of the blind 

 is as brilliant as that of the ocular side, and there are a few bright 

 spots on the blind side of the body as well. I find no difficulty in 

 attributing such a dull and partial pigmentation as is present on 

 the caudal fin of the Sole to the action of hght through the trans- 

 parent tissues of the fin, but the brilliant coloration of the Dab's 

 fins, associated, as it is, Mith a partial, if very slight, development 

 of colour on the non-transparent body, seems to require some 

 further explanation. 



Messrs. Cunningham and MacMuun find a difficulty in accepting 

 reversion or atavism as an explanation of the ambicolorate 

 condition, in that the hypothetical vertically swimming ancestor 

 of the Flat-fish must have " had an unpigmented white or silvery 

 ventral surface, as other symmetrical fishes have," whereas com- 

 pletely ambicolorate flat-fish are uniformly pigmented all over. 

 The difficulty certainly arises if we assume that the ancestor really 

 was paler on the ventral region than elsewhere ; but is it not 

 equally reasonable to assume a stage of evolution in which the 

 fish resembled such forms as Platax or Dasci/lhis, to take instances 

 from families widely separated from each other by systematists ? 

 Both forms have high compressed bodies, and in some species, at 

 any rate, in both genera the ventral region is as deeply pigmented 

 as the dorsal, E\en in tlie John Dory (Zeus faber), in which the 



