Species in the case of certain Fishes. 455 



rent. The cai-p and the bream are recognizable at once by 

 the great comparative basal extension of the dorsal in the 

 former, and the anal in the latter. The carp, which keeps 

 close to the bottom more constantly than the bream, possesses 

 barbels, while these arc wanting in thelattcr, which, on the otlier 

 hand, has the two lobes of the caudal pretty constantly unequal. 



The Chondrostoma (" nase ") and the tench, which, in 

 various points of view, constitute exceptions among our Cy- 

 prinidai, show us here, again, new modifications in the organs 

 relating to the mode of alimentation. Required generally to 

 take its food from beneath it, the nase, like our barbel, has the 

 mouth plainly inferior and the anal comparatively short ; but, 

 being destined to an almost exclusively vegetable diet, and 

 accustomed rather to graze upon, than to rout up the bottom, 

 it has no occasion for barbels ; and its lips are instead furnished 

 with a horny and trenchant sheath. Although willingly 

 keeping at the bottom, the tench, which is more omnivorous 

 than the carp and the barbel, and consequently requires to 

 take its nourishment in more varied positions, shows at the 

 same time a rather oblique mouth and a small lateral barbel ; 

 but in it the inferior fins are a little more powerful, and the 

 eye, in order to look in different directions, possesses a mobi- 

 lity and a facility of projection which does not occui in any 

 other of our Cyprinida. 



It would require a very great number of comparative obser- 

 vations to determine to what degree of dependence each of 

 these organs is subject, and which of them, under different 

 circumstances, is first called upon to vary. 



We might, I believe, push much further this comparative 

 investigation, which I now only indicate in passing. The 

 careful examination of the various dentitions, for example, has 

 often shown me an intimate and very natural relation between 

 the different forms of the teeth or of the pharyngeal plate, 

 wdiich betray the predominant nature of the diet, and a certain 

 modification of the internal or external framework, in view of 

 a peculiar gymnastic in the act of prehension. 



Our bleak {Alburnus lucidus), being especially insectivo- 

 rous, the habitual station of that fish, and the means of which 

 it must make use in order to obtain such or such a prey of 

 predilection, must vaiy, it would seem, with different condi- 

 tions and circumstances, and thereby exert more or less influ- 

 ence upon the form of the mouth, the sole organ of prehen- 

 sion. In connexion with this I have observed that the 

 majority of the bleak which inhabit certain of our rivers 

 present a deeper or more compressed form of body, a less 

 turned-up snout, and consequently a less oblique mouth than 



31* 



