450 Dr. V. Fatio on the Variability of the 



While the air-bladder, which presses against the left flank, 

 keeps the animal lying down, and it is by this means more 

 and more twisted, the eyes are subjected to very diftcrent 

 conditions and to very unequal use : the riglit eye, applied 

 to the bottom, remains in its normal vertical position with 

 regard to the axis of the head ; but the left eye, thus con- 

 demned to look always upward, turns more and more in order 

 to see round it as much as possible in a horizontal direction. 

 The fish has not been three months in its present position ; and 

 yet tiie globe of the eye, more and more elevated on the 

 frontal side, has already made more than one eighth of a turn, 

 or an angle of 45° to its normal position. Without desiring 

 to make too forced a comparison, one cannot help thinking of 

 the Pleuronectid fishes, which ordinarily repose upon one 

 side, and in which, as is well known, the two eyes, which are 

 at first symmetrically arranged, gradually come together, 

 during development, upon the same side of the animal. 



Lastly, from the study of the pathological case of this fish, 

 we may further draw a fresh proof of the fact, whicli has 

 already been several times demonstrated, that the icill is never 

 free, or that a deformation, even lohen accidental and ever so 

 small, seems always to he multiijlied, in the direction ofvaria- 

 hility, hy an unpremeditated xoill. In fact, if, after having 

 struggled in all directions to take its food, the rudd by chance 

 falls upon the left flank, the disagreeable pressure exerted by 

 the bottom upon the displaced air-bladder, and the instability 

 given to it by the convexity of this side of its body, invariably 

 uro-e the fish to quit this position (which nevertheless would 

 tend to reestablish equilibrium in its organism), and to make 

 effort after effbrt until it succeeds in replacing itself on its 

 ri"-ht flank, in a position which tends constantly more and 

 more towards deformation. 



Led by such data, either as to the effect of deformations of the 

 mouth, the head, and the body upon the air-bladder, or in- 

 versely as to the influence of the latter upon external form, 

 or as to the probable action of differences of pressure and 

 temperature upon the gas contained within the body of the 

 fish, I have lately, with the cooperation of M. Covelle, and 

 in one of his aquaria, twice made an experiment, which on 

 both occasions gave nearly identical results. 



We gradually warmed the whole mass of water in the 

 vessel, to see the cff'ect of temperature upon the relative posi- 

 tion of various fishes, some destitute of an air-bladder, others 

 provided with such an organ either closed or possessing a 

 communication with the exterior. The experiment was made 

 upon bullheads [Cottus gohio), perch {Percafuviatilis), tench 



