April 12th.—The lecture to-day was addressed especially to the ornithologists of the party, its object being to show how the same method of study,—that of testing the classification by the phases of growth in the different groups,—might be applied to the birds as profitably as to other types.
We have made good progress in the last forty-eight hours, and are fast leaving our friends “the trades” behind. The captain promises us smooth waters in a day or two. With the dying away of the wind will come greater heat, but as yet we have had no intensely warm weather. The sun, however, keeps us within doors a great part of the day, but in the evening we sit on the guards, watch the sunset over the waters, and then the moonlight, and so while away the time till nine or ten o’clock, when one by one the party disperses. The sea has been so rough that we have not been able to capture anything, but when we get into smoother waters, our naturalists will be on the look out for jelly-fish, argonautas, and the like.
April 13th.—In to-day’s lecture Mr. Agassiz returned again to the subject of geographical distribution and the importance of localizing the collections with great precision.
“As Rio de Janeiro is our starting-point, the water-system in its immediate neighborhood will be as it were a schoolroom for us during the first week of our Brazilian life. We shall not find it so easy a matter as it seems to keep our collections distinct in this region. The head-waters of some of the rivers near Rio, flowing in opposite directions, are in such close proximity that it will be difficult sometimes to distinguish them. Outside of the coast range, to which the Organ Mountains belong, are a number of short streams, little rills, so to speak, emptying directly into the ocean. It will be important to ascertain whether the same animals occur in all these short water-courses. I think this will be found to be the case, because it is so with corresponding small rivers on our northern coast. There are little rivers along the whole coast from Maine to New Jersey; all these disconnected rivers contain a similar fauna. There is another extensive range inland of the coast ridge, the Serra de Mantiquera, sloping gently down to the ocean south of the Rio Belmonte or Jequitinhonha. Rivers arising in this range are more complex; they have large tributaries. Their upper part is usually broken by waterfalls, their lower course being more level; probably in the lower courses of these rivers we shall find fishes similar to those of the short coast streams, while in the higher broken waters we shall find distinct faunæ.” The lecture closed with some account of the excursions likely to be undertaken in the neighborhood of Rio de Janeiro on arriving, and with some practical instructions about collecting, based upon Mr. Agassiz’s personal experience.[[11]]
April 14th.—Last evening was the most beautiful we have had since we left home; perfectly clear with the exception of soft white masses of cloud on the horizon, all their edges silvered by the moonlight. We looked our last for many months to come on the north star, and saw the southern cross for the first time. With the visible image I lost a far more wonderful constellation which had lived in my imagination; it has vanished with all its golden glory, a celestial vision as amazing as that which converted Constantine, and in its place stands the veritable constellation with its four little points of light.
The lecture to-day was upon the fishes of South America. “I will give you this morning a slight sketch of the characteristic fishes in South America, as compared with those of the Old World and North America. Though I do not know how the fishes are distributed in the regions to which we are going, and it is just upon the investigation of this point that I want your help, I know their character as distinguished from those of other continents. We must remember that the most important aim of all our studies in this direction will be the solution of the question whether any given fauna is distinct and has originated where it now exists. To this end I shall make you acquainted with the Brazilian animals so far as I can in the short time we have before beginning our active operations, in order that you may be prepared to detect the law of their geographical distribution. I shall speak to-day more especially of the fresh-water fishes.
“In the northern hemisphere there is a remarkable group of fishes known as the Sturgeons. They are chiefly found in the waters flowing into the Polar seas, as the Mackenzie River on our own continent, the Lena and Yenissei in the Old World, and in all the rivers and lakes of the temperate zone, communicating with the Atlantic Ocean. They occur in smaller numbers in most tributaries of the Mediterranean, but are common in the Volga and Danube, as well as in the Mississippi, in some of the rivers on our northern Atlantic and Pacific coasts, and in China. This family has no representatives in Africa, Southern Asia, Australia, or South America, but there is a group corresponding in a certain way to it in South America,—that of the Goniodonts. Though some ichthyologists place them widely apart in their classifications, there is, on the whole, a striking resemblance between the Sturgeons and Goniodonts. Groups of this kind, reproducing certain features common to both, but differing by special structural modifications, are called ‘representative types.’ This name applies more especially to such groups when they are distributed over different parts of the world. To naturalists the comparison of one of these types with another is very interesting, as touching upon the question of origin of species. To those who believe that animals are derived from one another the alternative here presented is very clear: either one of these groups grew out of the other, or else they both had common ancestors which were neither Sturgeons nor Goniodonts, but combined the features of both and gave birth to each.
“There is a third family of fishes, the Hornpouts or Bullheads, called Siluroids by naturalists, which seem by their structural character to occupy an intermediate position between the Sturgeons and Goniodonts. There would seem to be, then, in these three groups, so similar in certain features, so distinct in others, the elements of a series. But while their structural relations suggest a common origin, their geographical distribution seems to exclude it. Take, for instance, the Hornpouts; they are very few in the northern hemisphere, hardly ever occurring in those rivers where the Sturgeons abound, and they are very numerous in the southern hemisphere, in southern Asia, Australia, Africa, and South America, where the Sturgeons are altogether wanting. In South America the Siluroids everywhere exist with the Goniodonts, in all other parts of the world without them; the Goniodonts being only found in South America. If these were the ancestors of the Siluroids in South America, they were certainly not their ancestors anywhere else. If the Sturgeons were the ancestors of the Siluroids and of the Goniodonts, it is strange that their progeny should consist of these two families in South America, and in the Old World of the Siluroids only. But if all three had some other common ancestry, it would be still more extraordinary that its progeny should exhibit so specific a distribution upon the surface of our globe. The Siluroids lay very large eggs, and as they are very abundant in South America we shall no doubt have opportunities of collecting them. Of the reproduction of the Goniodonts absolutely nothing is known. Of course the embryology of both these groups would have a direct bearing on the problem of their origin.
“Another family very abundant in various parts of the world is that of the Perches. They are found all over North America, Europe, and Northern Asia; but there is not one to be found in the fresh waters of the southern hemisphere. In South America and in Africa they are represented however by a very similar group, that of the Chromids. These two groups are so much akin that from their structure it would seem natural to suppose that the Chromids were transformed Perches; the more so, since in the western hemisphere the latter extend from the high north to Texas, south of which they are represented by the Chromids. Here the geographical as well as the structural transition would seem an easy one. But look at the eastern hemisphere. Perches abound in Asia, Europe, and Australia, but there are no Chromids there. How is it that the Perches of this continent have been so fertile in producing Chromids, and the Perches of all other continents, except Africa, absolutely sterile in this respect? Or if we reverse the proposition, and suppose the Perches to have grown out of the Chromids, why have their ancestry disappeared so completely on the Asiatic side of the world, while they do not seem to have diminished on this? And if Perches and Chromids should be represented as descending from an older common type, I would answer that Palæontology knows nothing of such a pedigree.
“Next come the Chubs, or in scientific nomenclature the Cyprinoids. These fishes, variously called Chubs, Suckers, or Carps, abound in all the fresh waters of the northern hemisphere. They are also numerous in the eastern part of the southern hemisphere, but have not a single representative in South America. As the Goniodonts are characteristic of the southern hemisphere in its western half, so this group seems to be characteristic of it in its eastern half. But while the Cyprinoids have no representative in South America, there is another group there, structurally akin to them, called the Cyprinodonts. They are all small-sized; our Minnows belong to this group. From Maine to Texas they are found in all the short rivers or creeks all along the coast. It is for this reason that I expect to find the short coast rivers of South America abounding in Minnows. I remember to have found in the neighborhood of Mobile no less than six new species in the course of an afternoon’s ramble. These fishes are almost all viviparous, or at least lay their eggs in a very advanced state of development of the young. The sexes differ so greatly in appearance that they have sometimes been described as distinct species, nay, even as distinct genera.[[12]] We must be on our guard against a similar mistake. Here again we have two groups, the Cyprinoids and Cyprinodonts, so similar in their structural features that the development of one out of the other naturally suggests itself. But in South America there are no Cyprinoids at all, while the Cyprinodonts abound; in Europe, Asia, and North America on the contrary, the Cyprinoids are very numerous and the Cyprinodonts comparatively few.” The Characines were next considered with reference to their affinities as well as their geographical distribution; and a few remarks were added upon the smaller families known to have representatives in the fresh waters of South America, such as the Erythrinoids, the Gymnotines, &c. “I am often asked what is my chief aim in this expedition to South America? No doubt in a general way it is to collect materials for future study. But the conviction which draws me irresistibly, is that the combination of animals on this continent, where the faunæ are so characteristic and so distinct from all others, will give me the means of showing that the transmutation theory is wholly without foundation in facts.” The lecture closed with some account of the Salmonidæ, found all over the northern hemisphere, but represented in South America by the Characines, distinct species of which may be looked for in the separate water-basins of Brazil; and also of several other important families of South American fishes, especially the Osteoglossum, the Sudis, &c., interesting on account of their relation to an extinct fossil type, that of the Cœlacanths.[[13]]