456 Dr. V. Fatio on the Variahilty of the 



most of those which live more habitually in some of our lakes. 

 Now ill our transparent lakes (the lake of Geneva for ex- 

 ample) .we may very often see these graceful little Cyprinidge 

 hunting in numerous bands, and snapping up right and left 

 at the surface of the water the little insects of various sorts 

 that the winds or other accidents beat down upon it daily ; 

 whilst we less frequently observe these fishes at the very sur- 

 face in the moving, less transparent, shallower, and colder 

 waters of several of our streams, such as the Rhine for ex- 

 ample. It is difficult to avoid comparing these graceful little 

 fishes with the active swallows, which, like the bleak, so often 

 go in search of small insects close to the surface of the ground 

 or over the mirror of our lakes. We may fairly ask whether 

 meteorological influences, to a certain extent analogous with 

 those which impel the swallows alternately towards the ground 

 or the surface of the water, and to great altitudes in the air, 

 may not also, in different media, present the bleak with their 

 favourite food, according to circumstances, at the surface or at 

 a lower level in the water. 



According as the mouth, in order to adapt itself to the most 

 habitual circumstances in a given medium, becomes more or 

 less oblique, the back or the belly are, on the contrary, de- 

 pressed or elevated, at the same time that the body becomes 

 elongated or shortened. 



Heckel's theoretical line, which passes through the extremity 

 of the mouth and the middle of the caudal, displays these op- 

 posite deviations at the first glance, according as it passes at 

 a greater or less height with respect to the centre of the eye 

 and the summit of the back. The employment of this line 

 may be equally valuable in showing the degree of certain 

 deformations in fishes, as that of the two lines determining 

 the facial angle in other animals ; but it is a great pity that 

 Heckel and, in imitation of him, several ichthyologists have 

 too often attributed a specific value to the data obtained by 

 this mode of mensui'ation. 



It is easily understood that a modificatoiy influence like 

 that of which I have just spoken, however minute it may be, 

 but acting upon the individual from early youth, might in 

 time, and by multiplying itself by reproduction, affect a 

 species very profoundly under uncertain conditions. 



The action of the deformatory agents already mentioned 

 appears to me to be constant and regular enough ; however, 

 like every other rule, this also, I repeat, may present apparent 

 exceptions, which a conscientious study of the circumstances 

 and conditions of medium peculiar to each locality can alone 

 satisfactorily explain. 



