136 



Louis Agassiz. 



To Milne Edwards he wrote from here : " The 

 embryology and metamorphoses of the Chromides, 

 which I have just been studying, have convinced me 

 that the fishes with labyrinthic branchiae, separated 

 from all other fishes by Cuvier, as a family entirely 

 isolated on account of the strange structure of its 

 respiratory organs, are closely related to the Chro- 

 mides. Thus this group becomes, by its various 

 affinities, one of the most interesting of the class of 

 fishes, and the basin of the Amazon seems to be the 

 true home of this family. I will not fatigue you 

 with my ichthyological researches ; let me only add, 

 that the fishes are not uniformly spread over this 

 great basin. I have already acquired the certainty 

 that we must distinguish certain ichthyological 

 faunse very clearly characterised. Thus the species 

 inhabiting the river of Para, from the borders of the 

 sea to the mouth of the Tocantins, differ from those 

 which occur higher up ; those of the lower course of 

 the Xingu differ from those of the lower course of 

 the Tapajoz. Those of the numerous igarapes and 

 lakes of Manaos differ as much from those of the 

 principal course of the great river and of its great 

 affluents. It remains now to study the changes 

 which may take place in this distribution in the 

 course of the year, according to the height of the 

 waters, and perhaps also according to the epoch at 

 which the different species lay their eggs. Thus far 

 I have met but a small number of species having a 

 very extensive area of distribution. One of these is 

 the Studis gigas, found almost everywhere. It is the 

 most important fish of the river, that which, as food, 



