We are now fairly in the tropics. “The trades” blow heavily, and yesterday was a dreary day for those unused to the ocean; the beautiful blue water, of a peculiar metallic tint, as remarkable in color, it seemed to me, as the water of the Lake of Geneva, did not console us for the heavy moral and physical depression of sea-sick mortals. To-day the world looks brighter; there is a good deal of motion, but we are more accustomed to it. This morning the lecture had, for the first time, a direct bearing upon the work of the expedition. The subject was, “How to observe, and what are the objects of scientific explorations in modern times.”
“My companions and myself have come together so suddenly and so unexpectedly on our present errand, that we have had little time to organize our work. The laying out of a general scheme of operations is, therefore, the first and one of the most important points to be discussed between us. The time for great discoveries is passed. No student of nature goes out now expecting to find a new world, or looks in the heavens for any new theory of the solar system. The work of the naturalist, in our day, is to explore worlds the existence of which is already known; to investigate, not to discover. The first explorers, in this modern sense, were Humboldt in the physical world, Cuvier in natural history, Lavoisier in chemistry, La Place in astronomy. They have been the pioneers in the kind of scientific work characteristic of our century. We who have chosen Brazil as our field must seek to make ourselves familiar with its physical features, its mountains and its rivers, its animals and plants. There is a change, however, to be introduced in our mode of work, as compared with that of former investigators. When less was known of animals and plants the discovery of new species was the great object. This has been carried too far, and is now almost the lowest kind of scientific work. The discovery of a new species as such does not change a feature in the science of natural history, any more than the discovery of a new asteroid changes the character of the problems to be investigated by astronomers. It is merely adding to the enumeration of objects. We should look rather for the fundamental relations among animals; the number of species we may find is of importance only so far as they explain the distribution and limitation of different genera and families, their relations to each other and to the physical conditions under which they live. Out of such investigations there looms up a deeper question for scientific men, the solution of which is to be the most important result of their work in coming generations. The origin of life is the great question of the day. How did the organic world come to be as it is? It must be our aim to throw some light on this subject by our present journey. How did Brazil come to be inhabited by the animals and plants now living there? Who were its inhabitants in past times? What reason is there to believe that the present condition of things in this country is in any sense derived from the past? The first step in this investigation must be to ascertain the geographical distribution of the present animals and plants. Suppose we first examine the Rio San Francisco. The basin of this river is entirely isolated. Are its inhabitants, like its waters, completely distinct from those of other basins? Are its species peculiar to itself, and not repeated in any other river of the continent? Extraordinary as this result would seem, I nevertheless expect to find it so. The next water-basin we shall have to examine will be that of the Amazons, which connects through the Rio Negro with the Orinoco. It has been frequently repeated that the same species of fish exist in the waters of the San Francisco and in those of Guiana and of the Amazons. At all events, our works on fishes constantly indicate Brazil and Guiana as the common home of many species; but this observation has never been made with sufficient accuracy to merit confidence. Fifty years ago the exact locality from which any animal came seemed an unimportant fact in its scientific history, for the bearing of this question on that of origin was not then perceived. To say that any specimen came from South America was quite enough; to specify that it came from Brazil, from the Amazons, the San Francisco, or the La Plata, seemed a marvellous accuracy in the observers. In the museum at Paris, for instance, there are many specimens entered as coming from New York or from Pará; but all that is absolutely known about them is that they were shipped from those sea-ports. Nobody knows exactly where they were collected. So there are specimens entered as coming from the Rio San Francisco, but it is by no means sure that they came exclusively from that water-basin. All this kind of investigation is far too loose for our present object. Our work must be done with much more precision; it must tell something positive of the geographical distribution of animals in Brazil. Therefore, my young friends who come with me on this expedition, let us be careful that every specimen has a label, recording locality and date, so secured that it shall reach Cambridge safely. It would be still better to attach two labels to each specimen, so that, if any mischance happens to one, our record may not be lost. We must try not to mix the fishes of different rivers, even though they flow into each other, but to keep our collections perfectly distinct. You will easily see the vast importance of thus ascertaining the limitation of species, and the bearing of the result on the great question of origin.
“Something is already known. It is ascertained that the South American rivers possess some fishes peculiar to them. Were these fishes then created in these separate water-systems as they now exist, or have they been transferred thither from some other water-bed? If not born there, how did they come there? Is there, or has there ever been, any possible connection between these water-systems? Are their characteristic species repeated elsewhere? Thus we narrow the boundaries of the investigation, and bring it, by successive approaches, nearer the ultimate question. But the first inquiry is, How far are species distinct all over the world, and what are their limits? Till this is ascertained, all theories about their origin, their derivation from one another, their successive transformation, their migration from given centres, and so on, are mere beating about the bush. I allude especially to the fresh-water fishes, in connection with this investigation, on account of the precision of their boundaries. Looking at the matter theoretically, without a positive investigation, I do not expect to find a single species of the Lower Amazons above Tabatinga.[[6]] I base this supposition upon my own observations respecting the distribution of species in the European rivers. I have found that, while some species occur simultaneously in the many upper water-courses which combine to form the Rhine, the Rhone, and the Danube, most of them are not found in the lower course of these rivers; that, again, certain species are found in two of these water-basins and do not occur in the third, or inhabit only one and are not to be met in the two others. The brook trout, for instance (Salmo Fario), is common to the upper course and the higher tributaries of all the three river-systems, but does not inhabit the main bed of their lower course. So it is, also, and in a more striking degree, with the Salmling (Salmo Salvelinus). The Huchen (Salmo Hucho) is only found in the Danube. But the distribution of the perch family in these rivers is, perhaps, the most remarkable. The Zingel (Aspro Zingel) and the Schrætzer (Acerina Schrætzer) are only found in the Danube; while Acerina cernua is found in the Danube as well as in the Rhine, but not in the Rhone; and Aspro asper in the Danube as well as in the Rhone, but not in the Rhine. The Sander (Lucioperca Sandra) is found in the Danube and the other large rivers of Eastern Europe, but occurs neither in the Rhine nor in the Rhone. The common perch (Perca fluviatilis), on the contrary, is found both in the Rhine and Rhone, but not in the Danube, which, however, nourishes another species of true Perca, already described by Schaeffer as Perca vulgaris. Again, the pickerel (Esox Lucius) is common to all these rivers, especially in their lower course, and so is also the cusk (Lota vulgaris). The special distribution of the carp family would afford many other striking examples, but they are too numerous and too little known to be used as an illustration here.
“This is among the most remarkable instances of what I would call the arbitrary character of geographical distribution. Such facts cannot be explained by any theory of accidental dispersion, for the upper mountain rivulets, in which these great rivers take their rise, have no connection with each other; nor can any local circumstance explain the presence of some species in all the three basins, while others appear only in one, or perhaps in two, and are absent from the third, or the fact that certain species inhabiting the head-waters of these streams are never found in their lower course when the descent would seem so natural and so easy. In the absence of any positive explanation, we are left to assume that the distribution of animal life has primary laws as definite and precise as those which govern anything else in the system of the universe.
“It is for the sake of investigations of this kind that I wish our party to divide, in order that we may cover as wide a ground as possible, and compare a greater number of the water-basins of Brazil. I wish the same to be done, as far as may be, for all the classes of Vertebrates, as well as for Mollusks, Articulates, and Radiates. As we have no special botanist in the party, we must be content to make a methodical collection of the most characteristic families of trees, such as the palms and tree ferns. A collection of the stems of these trees would be especially important as a guide to the identification of fossil woods. Much more is known of the geographical distribution of plants than of animals, however, and there is, therefore, less to be done that is new in that direction.
“Our next aim, and with the same object, namely, its bearing upon the question of origin, will be the study of the young, the collecting of eggs and embryos. This is the more important, since museums generally show only adult specimens. As far as I know, the Zoölogical Museum at Cambridge is the only one containing large collections of embryological specimens from all the classes of the animal kingdom. One significant fact, however, is already known. In their earliest stages of growth all animals of the same class are much more alike than in their adult condition, and sometimes so nearly alike as hardly to be distinguished. Indeed, there is an early period when the resemblances greatly outweigh the differences. How far the representatives of different classes resemble one another remains to be ascertained with precision. There are two possible interpretations of these facts. One is that animals so nearly identical in the beginning must have been originally derived from one germ, and are but modifications or transmutations, under various physical conditions, of this primitive unit. The other interpretation, founded on the same facts, is, that since, notwithstanding this material identity in the beginning, no germ ever grows to be different from its parent, or diverges from the pattern imposed upon it at its birth, therefore some other cause besides a material one must control its development; and if this be so, we have to seek an explanation of the differences between animals outside of physical influences. Thus far both these views rest chiefly upon personal convictions and opinions. The true solution of the problem must be sought in the study of the development of the animals themselves, and embryology is still in its infancy; for, though a very complete study of the embryology of a few animals has been made, yet these investigations include so small a number of representatives from the different classes of the animal kingdom that they do not yet give a basis for broad generalizations. Very little is known of the earlier stages in the formation of hosts of insects whose later metamorphoses, including the change of the already advanced larva, first to the condition of a chrysalis and then to that of a perfect insect, have been carefully traced. It remains to be ascertained to what extent the caterpillars of different kinds of butterflies, for instance, resemble one another during the time of their formation in the egg. An immense field of observation is open in this order alone.
“I have, myself, examined over one hundred species of bird embryos, now put up in the museum of Cambridge, and found that, at a certain age, they all have bills, wings, legs, feet, &c., &c. exactly alike. The young robin and the young crow are web-footed, as well as the duck. It is only later that the fingers of the foot become distinct. How very interesting it will be to continue this investigation among the tropical birds!—to see whether, for instance, the toucan, with its gigantic bill, has, at a certain age, a bill like that of all other birds; whether the spoonbill ibis has, at the same age, nothing characteristic in the shape of its bill. No living naturalist could now tell you one word about all this; neither could he give you any information about corresponding facts in the growth of the fishes, reptiles, or quadrupeds of Brazil, not one of the young of these animals having ever been compared with the adult. In these lectures I only aim at showing you what an extensive and interesting field of investigation opens before us; if we succeed in cultivating even a few corners of it we shall be fortunate.”
In the evening, which is always the most enjoyable part of our day, we sat on the guards and watched the first tropical sunset we had yet seen. The sun went down in purple and gold, and, after its departure, sent back a glow that crimsoned the clouds almost to the zenith, dying off to paler rose tints on the edges, while heavy masses of gray vapor, just beginning to be silvered by the moon, swept up from the south.
April 7th.—To-day the lecture was upon the physical features of South America, something with reference to the geological and geographical work in which Mr. Agassiz hopes to have efficient aid from his younger assistants. So much of the lecture consisted of explanations given upon geological maps that it is difficult to record it. Its principal object, however, was to show in what direction they should work in order to give greater precision to the general information already secured respecting the formation of the continent. “The basin of the Amazons, for instance, is a level plain. The whole of it is covered with loose materials. We must watch carefully the character of these loose materials, and try to track them to their origin. As there are very characteristic rocks in various parts of this plain, we shall have a clew to the nature of at least some portion of these materials. My own previous studies have given me a special interest in certain questions connected with these facts. What power has ground up these loose materials? Are they the result of disintegration of the rock by ordinary atmospheric agents, or are they caused by the action of water, or by that of glaciers? Was there ever a time when large masses of ice descended far lower than the present snow line of the Andes, and, moving over the low lands, ground these materials to powder? We know that such an agency has been at work on the northern half of this hemisphere. We have now to look for its traces on the southern half, where no such investigations have ever been made within its warm latitudes; though to Darwin science is already indebted for much valuable information concerning the glacial phenomena of the temperate and colder portions of the South American continent. We should examine the loose materials in every river we ascend, and see what relation they bear to the dry land above. The color of the water in connection with the nature of the banks will tell us something. The waters of the Rio Branco, for instance, are said to be milky white; those of the Rio Negro, black. In the latter case the color is probably owing to the decomposition of vegetation. I would advise each one of our parties to pass a large amount of water from any river or stream along which they travel through a filter, and to examine the deposit microscopically. They will thus ascertain the character of the detritus, whether from sand, or lime, or granite, or mere river mud formed by the decomposition of organic matter. Even the smaller streams and rivulets will have their peculiar character. The Brazilian table-land rises to a broad ridge running from west to east, and determining the direction of the rivers. It is usually represented as a mountain range, but is in fact nothing but a high flat ridge serving as a water-shed, and cut transversely by deep fissures in which the rivers flow. These fissures are broad in their lower parts, but little is known of their upper range; and whoever will examine their banks carefully will do an important work for science. Indeed, very little is known accurately of the geology of Brazil. On the geological maps almost the whole country is represented as consisting of granite. If this be correct, it is very inconsistent with what we know of the geological structure of other continents, where the stratified rocks are in much larger proportions.”
Upon this followed some account of the different kinds of valley formation and of terraces. “Do the old terraces above the rivers of South America correspond to the river terraces on any of our rivers,—those of the Connecticut, for instance,—showing that their waters had formerly a much greater depth and covered a much wider bottom? There must of course have been a cause for this great accumulation of water in ancient periods. I account for it in the northern half of the hemisphere by the melting of vast masses of ice in the glacial period, causing immense freshets. There is no trustworthy account of the river terraces in Brazil. Bates, however, describes flat-topped hills between Santarem and Pará in the narrow part of the valley, near Almeyrim, rising 800 feet above the present level of the Amazons. If this part of the valley were flooded in old times, banks might have been formed of which these hills are a remnant. But because such a theory might account for the facts it does not follow that the theory is true. Our work must be to study the facts, to see, among other things, of what these hills are built, whether of rock or of loose materials. No one has told us anything as yet of their geological constitution.”[[7]]