238 A JOURNEY IN BRAZIL. 
zonian species, before taking on its own characteristic 
features, passes through a stage resembling the perma- 
nent adult condition of the Hemirhamphus. It is interest- 
ing to find that animals, which have their natural homes so 
far from each other that there is no possibility of any ma- 
terial connection between them, are yet so linked together by 
structural laws, that the development of one species should 
rocall the adult form of another.* The story of the Acaras, 
* When I attempted to record my impression of the basin of the Amazons, 
and characterized it as a fresh-water ocean with an archipelago of islands, I 
did not mean to limit the comparison to the wide expanse of water and the 
large number of islands. The resemblance extends much further, and the 
whole basin may be said to be oceanic also, in fhe character of its fauna. It is 
true, we are accustomed to consider the Chromides, the Characines, the Silu- 
roids, and the Goniodonts, which constitute the chief population of this net- 
work of rivers, as fresh-water fishes ; but in so doing we shut our eyes to their 
natural affinities, and remember only the medium in which they live. Let any 
one enter upon a more searching Comparison, and he will not fail to perceive 
that, under the name of Chromides, fishes are united which in their form and 
general appearance recall several families of the class, only known as inhab- 
itants of the sea. The genus Pterophyllum, for instance, might be placed side 
by side with the Chsetodonts, without apparently violating its natural affini- 
ties, since even Cuvier considered it as a Platax. The genera Symphysodon 
and Uaru would not seem very much out of place, by the side of Brama. The 
genus Geophagus and allied forms recall at once the Sparoids, with which 
some of them were associated by earlier ichthyologists ; while the genus 
Crenicichla forms a striking counterpart to the genus Malacanthus. Finally, 
the genus Acara and their kindred closely resemble the Pomacentroids. In- 
deed, had not the fresh-water genera Pomotis, Centrarchus, and the like, been 
erroneously associated with the Percoids, the intimate relations which bind 
them to the Chromides, and these again to the marine types mentioned above, 
would long ago have been acknowledged. The genus Monocirrus is a minia- 
ture Toxotcs, with a barbel. Polycentrus, which is also found in the Ama- 
zons, stands nearest to Acara and Heros ; it has only a larger number of 
anal spines In this connection it ought not to be overlooked that these 
fishes are not pelagic, like the Scomberoids, but rather archipelagic, if I 
may use this word to designate fishes dwelling among low islands. If 
we discard the long-prevailing idea of a close relationship between the 
