454 Dr. V, Fatio on the Variability of the 



hension of food above, in front of, or beneath the individual, 

 the form of the mouth also varies more or less in the fishes 

 which generally live between these two extremes or in mid- 

 water. Lastly, as corollaries of these first modifications de- 

 pendent on habitat, I may recall the gradual apparition at the 

 sides of the mouth, in our bottom -feeding Cyprinidae, of tactile 

 organs, more or less developed barbels. It must not be for- 

 gotten that, notwithstanding its constant communication with 

 the exterior, the air-bladder, which is a little variable in posi- 

 tion and proportions, may here exert an influence, up to a 

 certain point, upon the general form of the fish and its mode 

 of gymnastic, by pressing more or less against one part or 

 another of the individual. Under the influence of the agents 

 which superinduce the transformations of the mouth, we also 

 see appear other correlative modifications in various parts of 

 the animal — among others, in the greater or less declivity 

 of the head, in the more or less convex or depressed form of 

 the back and belly, in the variable compression of the sides, 

 in the situation and proportions of the eye with regard to the 

 forehead, and finally in the relative position and the develop- 

 ment of certain fins. 



These various tendencies to adaptation may be, I repeat, 

 very different in other families, in which the equilibrium of the 

 organism rests upon other foundations ; or they may be ac- 

 companied by new modifications affecting other parts, such as 

 the nature of the integuments for example. 



Our barbel, which chiefly seeks its nourishment beneath it, 

 on the bottom or in the mud, has the mouth opening below 

 and furnished with barbels, the eye comparatively small, and 

 the base of the anal fin rather short ; the bleak, which, on the 

 contrary, most frequently snatches its prey at the surface or 

 above it, has the mouth oblique, opening more or less up- 

 wards, and destitute of barbels, with a large eye and the 

 base of the anal fin comparatively elongated. The rudd and 

 the roach, which most commonly seek their food at midwater, 

 although the mouth is oblique in the former and quasi-hori- 

 zontal in the latter, and without barbels in both, have never- 

 theless the anal and dorsal fins of nearly equal importance, 

 and a body usually rather deeper than the species above indi- 

 cated as inhabiting extreme situations. A certain resemblance 

 of general form (which, however, is variable for each of these 

 species in different media) may be due to a similitude of 

 habitat in an average medium ; but the examination of the 

 grinding-plate and of the pharyngeal teeth betrays a marked 

 preference for aliments of different natures, and consequently 

 modes of preliension which are probably also somewhat diffe- 



