OBSERVATIONS AND CORRESPONDENCE. 



Professor Giard on the Evolution of Flat-Fishes. 



Professor Giard replies to my article on " The Evolution of Flat-Fishes " as 

 if I had commenced the controversy between us by making an attack on him. The 

 fact is, on the contrary, that my criticism of his note on an abnormal turbot was a 

 defence of my experiments as described in the Zoologischer Anzeiger against his 

 attempt to prove that they were superfluous on the one hand and inconclusive on the 

 •other. The Professor deliberately maintained that the existence cf specimens of 

 Rhombus maximus in which the head was in certain respects monstrous or abnormal, 

 and in which the blind side was pigmented all over like the eyed side, was in itself a 

 sufficient proof that the coloration of the upper sides of flat-fishes was due to the 

 direct action of light, and that pigment was produced on the lower sides of such 

 fishes when those sides were exposed to light. My opponent still refuses to admit 

 the cogency of my arguments and the validity of my defence, and I will try, there- 

 fore, to answer his objections. 



Professor Giard distinguishes between what he calls monstrous Pleuronectids 

 which show arrested development and the double flat-fishes whose existence he says 

 I stated to be recorded in numerous works on Ichthyology. The distinction is valid, 

 and I have never ignored it : there are ambicolorate specimens which are normal in 

 all respects except colour, and there are ambicolorate specimens in which the head 

 and eyes have the abnormality described in Giard's specimen. But what I said was, 

 that the " monstrous Pleuronectids which show arrested development without stop- 

 page in growth," to use the Professor's words, and not merely ambicolorate speci- 

 mens, were recorded and described in a large number of ichthyological treatises, and 

 that the specimens recorded did not belong exclusively to the one species Rhombus 

 maximus, as Professor Giard stated, but to other species and other genera as well. 

 So much he now admits, citing numerous cases in literature. He refers to a speci- 

 men of the turbot described by Couch, and quotes that author as stating that the 

 same monstrosity is not very rare in Zeiigopferus punctatus. I i^egret that he has not 

 given volume and page for these references to Couch, for in Couch's general' work 

 on " Fishes of the British Islands," vol. iii., I find a description and figure of the 

 monstrosity in question in the flounder, Plcnronectes flesus, but no mention of it in 

 connection with Rhombus or Zeugopteriis. 



But in correcting his original statement Professor Giard says the monstrosity 

 occurs among the flounders, " whose development is as slow as that of the turbot," 

 and that it may be stated in a general way to occur among the Pleuronectidae of 

 gradual metamorphosis (palingenetic development). I confess I cannot fully 

 understand these statements, but so far as they convey a meaning to me I entirely 

 ■disagree with them. The exact age of the turbot and flounder at the beginning and 

 end of metamorphosis, and the exact time occupied in that process, has never, so far as 

 I know, been ascertained. But it is certain that the development of the turbot and 

 brill is in some respects peculiar and different not only from that of the flounder, 

 but from that of all other Pleuronectidae. The two species of Rhombus 

 have a well-developed air-bladder during their metamorphosis, and, in conse- 

 quence of this, swim near the surface of the sea until the metamorphosis is com- 

 plete, or nearly so ; they also attain an unusually large size before they 

 acquire the adult characters. I have specimens of the turbot 36 cm. in 

 length in which the transformation is by no means finished. The flounder, 

 on the other hand, and all species of the genus Pleuronectes have no air-bladder 

 ■during metamorphosis, and begin to lie on the bottom at the very commence- 

 ment of that process. The flounder has acquired the characters of the adult 



