On reaching Manaos, at the junction of the Rio Negro and the Amazons, I had already collected more than three hundred species of fishes, half of which have been painted from life, that is, from the fish swimming in a large glass tank before my artist. I am often pained to see how carelessly colored plates of these animals have been published. Not only have we tripled the number of species, but I count new genera by dozens, and I have five or six new families for the Amazons, and one allied to the Gobioides entirely new to Ichthyology. Among the small species especially I have found novelties. I have Characines of five or six centimetres and less, adorned with the most beautiful tints, Cyprinodonts resembling a little those of Cuba and the United States, Scomberesoces allied to the Belone of the Mediterranean, a considerable number of Carapoides, and Rays of different genera from those of the ocean, and therefore not species which ascend the river; and a crowd of Goniodonts and Chromides of unpublished genera and species. But what I appreciate most highly is the facility I have for studying the changes which all these fishes undergo with age and the differences of sex among them; which are often very considerable. Thus I have observed a species of Geophagus in which the male has a very conspicuous protuberance on the forehead, wholly wanting in the female and the young. This same fish has a most extraordinary mode of reproduction. The eggs pass, I know not how, into the mouth, the bottom of which is lined by them, between the inner appendages of the branchial arches, and especially into a pouch, formed by the upper pharyngials, which they completely fill. There they are hatched, and the little ones, freed from the egg-case, are developed until they are in a condition to provide for their own existence. I do not yet know how long this continues; but I have already met with specimens whose young had no longer any vitelline sac, but were still harbored by the progenitor. As I shall still pass a month at Teffé I hope to be able to complete this observation. The examination of the structure of a great number of Chromides has led me to perceive the affinities between these fishes and several other families with which we have never thought of associating them. In the first place, I have convinced myself that the Chromides, formerly scattered among the Labroides and the Sciænoids, really constitute a natural group recognized nearly at the same time and in an independent manner by Heckel and J. Müller. But, beside these, there are the genera Enoplosus, Pomotis, Centrarchus, and some other neighboring genera, classed among the Percoids by all Ichthyologists, which seem to me, from this distance and without means of direct comparison, so near the Chromides that I do not see how they can be separated, especially now that I know the lower pharyngials not to be invariably soldered in the Chromides. And then the embryology and metamorphoses of the Chromides, which I have just been studying, have convinced me that the fishes with labyrinthic branchiæ, 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 Chromides. Thus this group becomes, by its various affinities, one of the most interesting of the class of fishes, and the basin of the Amazons 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 several ichthyological faunæ very clearly characterized. Thus the species inhabiting the river of Pará, from the borders of the sea to the mouth of the Tocantins, differ from those which are met in the network of anastomoses uniting the river of Pará with the Amazons proper. The species of the Amazons below the Xingu 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 igarapés 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 those is the Sudis gigas, found almost everywhere. It is the most important fish of the river, that which, as food, corresponds to cattle for the population along the banks. Another problem to be solved is, how far this phenomenon of the local distribution of fishes is repeated in the great affluents of the Amazons. I shall try to solve it in ascending the Rio Negro and Rio Madeira, and as I return to Manaos I shall be able to compare my first observations in this locality with those of another season of the year. Adieu, my dear friend. Remember me to M. Elie de Beaumont and to those of my colleagues of the Academy who are interested in my present studies. My kind remembrance also to your son.
Always yours,
L. Agassiz.
[70]. We found that this information was incorrect, at least for some species, as will be seen hereafter. I let the statement stand in the text, however, as an instance of the difficulty one has in getting correct facts, and the danger of trusting to the observations even of people who mean to tell the real truth. No doubt some of these Acaras do occasionally deposit their young in the sand, and continue a certain care of them till they are able to shift for themselves. But the story of the fisherman was one of those half truths as likely to mislead, as if it had been wholly false. I will add here a few details concerning these Acaras, a name applied by the natives to all the oval-shaped Chromides. The species which lay their eggs in the sand belong to the genera Hydrogonus and Chætobranchus. Like the North American Pomotis, they build a kind of flat nest in the sand or mud, in which they deposit their eggs, hovering over them until the young are hatched. The species which carry their young in the mouth belong to several genera, formerly all included under the name of Geophagus by Heckel. I could not ascertain how the eggs are brought into the mouth, but the change must take place soon after they are laid, for I have found in that position eggs in which the embryo had just begun its development as well as those in a more advanced stage of growth. Occasionally, instead of eggs, I have found the cavity of the gills, as also the space enclosed by the branchiostegal membrane, filled with a brood of young already hatched. The eggs before hatching are always found in the same part of the mouth, namely, in the upper part of the branchial arches, protected or held together by a special lobe or valve formed of the upper pharyngeals. The cavity thus occupied by the eggs corresponds exactly to the labyrinth of that curious family of fishes inhabiting the East Indian Ocean, called Labyrinthici by Cuvier. This circumstance induces me to believe that the branchial labyrinth of the eastern fishes may be a breeding pouch, like that of our Chromides, and not simply a respiratory apparatus for retaining water. In the Amazonian fish a very sensitive network of nerves spreads over this marsupial pouch, the principal stem of which arises from a special nervous ganglion, back of the cerebellum, in the medulla oblongata. This region of the central nervous system is strangely developed in different families of fishes, and sends out nerves performing very varied functions. From it arise, normally, the nerves of movement and sensation about the face; it also provides the organs of breathing, the upper part of the alimentary canal, the throat and the stomach. In the electric fishes the great nerves entering the electric battery arise from the same cerebral region, and now I have found that the pouch in which the egg of the Acara is incubated and its young nursed for a time, receives its nerves from the same source. This series of facts is truly wonderful, and only shows how far our science still is from an apprehension of the functions of the nervous system.—L. A.
[71]. It is a very general habit among the South American Indians to pierce the nose, ears, and lips with holes, in which they hang pieces of wood and feathers, as ornaments.
[72]. See “Expeditions into the Valley of the Amazons,” published by the Hakluyt Society.
[73]. This species belongs to one of the subdivisions of the genus Auchenipterus; it is undescribed, and Mr. Burkhardt has made five colored sketches of a number of specimens of different sizes, varying in their markings.—L. A.
[74]. 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 the character of its fauna. It is true, we are accustomed to consider the Chromides, the Characines, the Siluroids, and the Goniodonts, which constitute the chief population of this network 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 inhabitants of the sea. The genus Pterophyllum, for instance, might be placed side by side with the Chætodonts, without apparently violating its natural affinities, 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. Indeed, 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 miniature Toxotes, with a barbel. Polycentrus, which is also found in the Amazons, 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 Characines and Salmonides, based solely upon the presence of an adipose fin, we may at once perceive how manifold are the affinities between the Characines on one hand, and on the other the Scopelines and Clupeoids, all of which are essentially marine. These relations may be traced to the details of the genera; Gasteropelecus, from the family of Characines, is the pendant of Pristigaster among the Clupeoids, as Chalcinus recalls Pellona. In the same way may Stomias and Chauliodus be compared to Cynodon and the like; or Sudis and Osteoglossum to Megalops, and Erythrinus to Ophicephalus, &c., &c. The Goniodonts may at first sight hardly seem to have any kindred among marine fishes; but if we take into account the affinity which unquestionably links the genus Loricaria and its allies with Pegasus, and further remember that to this day all the ichthyologists, with the sole exception of C. Duméril, have united Pegasus in one order with the Pipe-fishes, it will no longer be doubted that the Goniodonts have at least a remarkable analogy with the Lophobranches, if they should not be considered as bearing a close structural relation to them. But this relation truly exists. The extraordinary mode of rearing their young, which characterizes the various representatives of the old genus Syngnathus, is only matched by the equally curious incubation of the eggs in Loricaria. And as to the other families represented in the basin of the Amazons, such as the Skates, the Sharks, the Tetraodonts, the Flat-fishes (Pleuronectides), the Bill-fishes (Scomberesoces), the Anchovis, Herrings, and other forms of the family of Clupeoids, the Murænoids, the genuine Sciænoids, the Gobioids, &c., &c., they are chiefly known as marine types; while the Cyprinodonts occur elsewhere both in salt and fresh water. The Gymnotines are thus far only known as fresh-water fishes, nor do I see any ground for comparing them to any marine type. They cannot be compared to the Murænoids, with which they have thus far been associated. The only real affinity I can trace in them is with the Mormyri of the Nile and Senegal, and with the Notopteri of the Sunda Islands. Eel-shaped fishes are by no means all related to one another, and their elongated form, with a variety of patterns, is no indication of their relationship. It may, nevertheless, be inferred from what precedes, that the fishes of the Amazons have, as a whole, a marine character peculiarly their own, and not at all to be met with among the inhabitants of the other great rivers of the world.
These peculiarities extend to other classes besides fishes. Among the Bivalve shells, it has long been known that the Amazons nourishes genera of Naiades peculiar to its waters, or only found besides in the other great rivers of South America; such as Hyria, Castalia, and Mycetopus, to which I would add another genus, founded upon slender, sickle-shaped Unios, common to North and South America. But what seems to have escaped the attention of conchologists is the striking resemblance of Hyria and Avicula, of Castalia and Arca, of Mycetopus and Solen, &c. Thus exhibiting another repetition of marine types in a family exclusively limited to fresh waters, and having structural characters of its own, entirely distinct from the marine genera, the appearance of which they so closely ape. In this connection I cannot suppress the remark, that it would be puerile to consider such mimicry as indicative of a community of origin. Some of the land shells even recall marine forms; such are some of the Bulimus tribe, which resemble the genus Phasianella and Littorina far more than their own relatives. The similarity of the fringes of the anterior margin of the foot is particularly striking. The Ampullariæ remind one also, in a measure, of the marine genera Struthiolaria, Natica, &c., and many fossils of the latter family have been confounded with fresh-water Ampullariæ.
The most noticeable feature of the Amazonian fauna, considered with reference to its oceanic character, is, however, the abundance of Cetaceans through its whole extent. Wherever I have navigated these waters, from Pará, where the tides still send the salt brine up the river, to Tabatinga on the borders of Peru, in all the larger and smaller tributaries of the great stream as well as in the many lakes connected with their ever-changing course, I have seen and heard them, gamboling at the surface and snoring rhythmically, when undisturbed in their breathing. At night, especially, when quietly at anchor in the river, you hardly ever fail to be startled by the noise they make, when reaching the surface to exhale forcibly the air they have long retained in their lungs while under water. I have noticed five different species of this order of animals in the waters of the Amazons, four of which belong to the family of Porpoises and one to that of Manatees. Mr. Burkhardt has drawn three of them from fresh specimens for me, and I hope before long to secure equally faithful representations of the others, when I shall describe them all comparatively. One of the Porpoises belongs to the genus Inia, and may be traced on the upper tributaries of the Amazons to Bolivia, another resembles more our common Porpoise, while still another recalls the Dolphin of the sea-coast; but I have been unable to ascertain whether any one of them is identical with the marine species. At all events, the black Porpoise of the bay of Marajo, frequently seen in the vicinity of Pará, is totally different from the gray species seen higher up the stream.—L. A.