species in the case of certain Fishes. 447 



diffei'ent characters that the more or less ratiotial establishment 

 of genera and species in great part depends. 



The great functions of life, nutrition and reproduction, 

 naturallj govern this selection of the more or less influencible 

 parts. According as the preservation of the individual or the 

 perpetuation of the species is brought into question by the 

 changes of medium, it is evidently also among the external 

 organs which serve one or the other that the modifiable cha- 

 racter the best fitted to effect the adaptation will be selected. 

 The degree of complaisance, or, on the contrary, the exigen- 

 cies of these two essential functions, allow more or less latitude 

 to such or such an organ which brings them directly into 

 relation with the outer world. 



Although only considering the question from one of its 

 sides, and devoting ourselves more particularly to the inves- 

 tigation of certain parts specially useful for the preservation 

 of the individual, we nevertheless cannot here help recog- 

 nizing, as it were, a brake imposed upon the too rapid modi- 

 fications of one organ by the exigencies of anothei — may Je, as 

 a new struggle in view of a more or less stable equilibrium^ 

 until perfect adaptation. 



As I have said, the whole organism of an individual must 

 be able to bend itself to the more or less sudden changes ne- 

 cessitated in the modification of gestures or habits by the 

 apparition of a new exigency, and follow in an equilibrated 

 manner the transformations effected in the organ of relation 

 which was first called upon to vary. 



If we were to select as examples of struggle between exter- 

 nal and internal organs in certain fishes, on the one hand, the 

 eye or the mouth as displaying the appetites, and, on the 

 other, the air-bladder as above all subjected to the conditions 

 of temperature or pressure of the medium, we should soon find 

 several curious cases of ruptures of equilibrium, both acci- 

 dental and normal, and injurious, sometimes to the individual, 

 sometimes to the species. 



Among others, we know that in the perch {Perca fluviatilis) , 

 suddenly drawn up by the line from the depths of our lakes, 

 the air-bladder, being too rapidly transported from a consider- 

 able pressure to a much smaller one, is suddenly distended in 

 an extraordinaiy manner, projecting itself into the mouth, and 

 sometimes even driving out a part of the digestive organs. 

 We also know that the pike {Esox lucius), when carried, by 

 its voracity, too rapidly from the deeper towards the superficial 

 strata of the water in pursuit of prey, is forcibly retained at 

 the surface by an exaggerated development of the swimming- 

 bladder, and often perishes in consequence of this injury, 

 which in this case is quite voluntary. 



