up the Rio Negro. 



133 



at certain times. This discovery so fascinated Agassiz 

 that he made his headquarters here for a month, 

 carefully studying the fish, which he named after 

 the Emperor, and which is described in the following 

 letter to his Majesty : 



Sire : — On arriving here this morning I had the 

 most agreeable and unexpected surprise. The first 

 fish brought to me was the Acara [see accompany- 

 ing cut], which your Majesty kindly permitted me to 

 dedicate to you, and by an unlooked-for good for- 

 tune it was the breeding season, and it had its mouth 

 full of little young ones in the process of develop- 

 ment. Here, then, is the most incredible fact in 

 embryology fully confirmed, and it remains for me 

 only to study, in detail and at leisure, all the changes 

 which the young undergo up to the moment when 

 they leave their singular nest, in order that I may 

 publish a complete account of this curious history. 

 My anticipations as to the distribution of fishes are 

 confirmed ; the river is inhabited by several very dis- 

 tinct ichthyological faunae, which have, as a common 

 link, only a very small number of species to be met 

 with everywhere. It remains now to ascertain with 

 precision the limits of these ichthyological regions, 

 and I may perhaps be drawn on to devote some time 

 to this study, if I find the means of accomplishing it. 

 There is a question which now becomes very inter- 

 esting ; it is to know how far the same phenomenon 

 is reproduced in each one of the great affluents of 

 the river Amazon, or, in other words, whether the 

 fishes of the upper regions of the Rio Madeira, the 



