Species in the case of certain Fishes. 457 



It is always difficult to determine what is the preponderant 

 influence, and consequently in what direction the first modifi- 

 cations will take place. I can easily understand the error of 

 Blanchard, who has distinguished specifically, under the name 

 of Alburnus mirandella, our elongated bleak of the lake of 

 Geneva from the deeper ones of the French rivers. Never- 

 theless I cannot yet so easily explain the causes of the com- 

 paratively deeper form of the bleak which Heckel originally 

 and erroneously distinguished under the name of Alburnus 

 lacustris from the Neusiedler and Flatten lakes, as I do not 

 sufficiently know the nature and relative importance of the 

 conditions of medium proper to those two lakes. 



In fact the modificative and conservative agents opposed to 

 each other may be of very diverse natures. Under influences 

 of medium or local conditions we must include, in the case 

 of our fishes, the depth of the medium, the degree of pressure, 

 the transparency or the possible light, the surrounding tem- 

 perature, the nature and origin of the water, the composition 

 of the bottom, the faunas and floras of the region, the climate 

 or the usual meteorological conditions of the locality, and, 

 lastly, many other circumstances which are often difficult to 

 appreciate. 



I may here refer to the case of the Leuciscus rutilus of the 

 Bruniger lake, of which I spoke in the number of the ' Biblio- 

 th^que' for September 1876, and which, in consequence of the 

 retreat of the water of this little basin upon an almost entirely 

 rocky bottom, was compelled to go to the surface in search of 

 the vegetable and animal debris which were carried there by 

 the winds. I stated that the body of this fish had by degrees 

 become more, elongated, with a very pale coloration, and that 

 the mouth had acquired a more oblique position. 



If I have dwelt so much upon this side of the variability 

 of our fishes, and in particular of our bleak, it is because ana- 

 logous cases, sometimes wrongly interpreted, also occur fre- 

 quently in other genera, and have very often served for the 

 establishment of numerous false species. 



From all that has been stated, it would seem that we may 

 derive, on the one hand, fresh proofs in support of the con- 

 stant variability of the species under a concourse of favourable 

 circumstances, and, on the other, the indication of certain 

 limits imposed upon the modifications possible in a given 

 direction under the influence of a peculiar, too predominant 

 condition. In other words, it appears that in default of 

 sufficient time, or of a relative equilibrium in the diffe- 

 rent influences of the medium, the series of correlative modi- 

 fications cannot be effected in a durable manner, and that 



