.g,, OBSERVATIONS AND CORRESPONDENCE. 637 



due to an error of observation, basing this reply not, as the Professor says, on the 

 observation of an adult double plaice, which has nothing to do with the case of a 

 larva, but on the repeated observation of dozens of lar\'al turbot at the stage of 

 metamorphosis described by Professor Mcintosh. But I find, on referring to 

 the passage in question (Trans. Roy. Sac. Ediiib., vol. xxxv., pt. iii., p. 846), that I 

 owe an apology to Professor Mcintosh. The error is not an error of observation on 

 iis part, but an error of citation on the part of Professor Giard. Professor 

 Mcintosh does not state that the larval specimen in question swam constantly in the 

 •vertical position. He does not state that he saw it alive. All he says in reference 

 to its position is as follows : " The appearance of certain spines on both sides after 

 the right eye is at the edge, indicates the possibility that for some time the fish may 

 ■occasionally resume the vertical position in swimming." 



I must mention here, lest I should be suspected of omitting anything, that the 

 only other citation of Professor Mcintosh by Professor Giard, namely, from the 

 ^' Marine Invertebrata, etc., of St. Andrew's," refers to a young turbot which had an 

 eye on each side, and in which the dorsal fin did not form any hooked process, and 

 which was, therefore, quite different from the typical form of arrested development 

 binder consideration. 



Professor Giard states that the double plaice which I described in my article as 

 being under my observation in captivity was not monstrous and only imperfectly 

 double. I stated that it was monstrous in Professor Giard's sense : that the right 

 •eye was on the edge of the head and the dorsal fin terminated behind that eye, though 

 it does not form so prominent a hook as in other cases. What excuse is there to be 

 made for the Professor when he thus asserts that my specimen has the head and 

 eyes normal, although I expressly described the head and eyes as abnormal ? 



Professor Giard now admits that ambicolorate specifnens with normal eyes lie 

 liorizontally on the ground. That being so, how can an ambicolorate specimen, 

 ■with an abnormal head, be a proof of the direct action of the light in producing 

 pigment on the side where it is normally absent ? 



Professor Giard now affirms not that the monstrous specimens swim all their 

 3ife vertically, but that they remain longer in that position than others, in any case 

 long enough to allow the influence of the light to act efficaciously on the side 

 ordinarily colourless. But he has less evidence, if possible, to support this than he 

 had to support his original assumption. My contention has been throughout that 

 •evidence is required on the question of the relation of pigmentation to light. I 

 f)roduced some evidence in the published result of my experiment. Professor Giard 

 'has failed to appreciate that evidence ; he has produced no evidence himself, but, 

 while recording a specimen of a familiar abnormality, has attempted to maintain 

 ■that the existence of this abnormality renders my evidence superfluous. He accuses 

 me of an astonishing misconception of the laws of cross-breeding. The miscon- 

 ception was not mine, for I simply pointed out the necessary consequences of the 

 ■action of inheritance on the colour of flounders as conceived by himself. Supposing 

 we accept his correction, what is the application to the results of my experiment ? 

 The progeny cannot be intermediate, he says, between two progenitors, but must 

 resemble closely one or the other. Therefore, the progeny of a right-sided and a 

 left-sided flounder must be either right-sided or left-sided. But the flounders in my 

 experiment were coloured on both sides, and, therefore, the possibility of the occur- 

 rence of reversed examples among their ancestors had nothing whatever to do with 

 their condition, as Giard originally suggested. 



This controversy was not of my seeking, and I can only regret that Professor 

 -Giard should have been betrayed in discussing this subject into a temporary 

 departure from the accuracy and caution which he usually exhibits. Any zoologist 

 who reads the literature must admit that satisfactory evidence as to the causes 

 which produce abnormalities of colour and structure in flat-fishes is not at present 

 available. That the typical ambicolorate monstrosity in which the metamorphosis 

 of the head is permanently arrested is produced by the action of light, or by the 

 direct action of conditions at all, is a pure assumption unsupported by any evidence, 

 and, therefore, to maintain that the occurrence of such specimens is a proof that pig- 

 mentation of the skin is dependent upon light, or its absence on the absence of light, 



