To-day we have seen numbers of flying-fish from the deck, and were astonished at the grace and beauty of their motion, which we had supposed to be rather a leap than actual flight. And flight indeed it is not, their pectoral fins acting as sails rather than wings, and carrying them along on the wind. They skim over the water in this way to a great distance. Captain Bradbury told us he had followed one with his glass and lost sight of it at a considerable distance, without seeing it dip into the water again. Mr. Agassiz has great delight in watching them.[[8]] Having never before sailed in tropical seas, he enjoys every day some new pleasure.
April 9th.—Yesterday Mr. Agassiz lectured upon the traces of glaciers as they exist in the northern hemisphere, and the signs of the same kind to be sought for in Brazil. After a sketch of what has been done in glacial investigation in Europe and the United States, showing the great extension of ice over these regions in ancient times, he continued as follows: “When the polar half of both hemispheres was covered by such an ice shroud, the climate of the whole earth must have been different from what it is now. The limits of the ancient glaciers give us some estimate of this difference, though of course only an approximate one. A degree of temperature in the annual average of any given locality corresponds to a degree of latitude; that is, a degree of temperature is lost for every degree of latitude as we travel northward, or gained for every degree of latitude as we travel southward. In our times, the line at which the average annual temperature is 32°, that is, at which glaciers may be formed, is in latitude 60° or thereabouts, the latitude of Greenland; while the height at which they may originate in latitude 45° is about 6,000 feet. If it appear that the ancient southern limit of glaciers is in latitude 36°, we must admit that in those days the present climate of Greenland extended to that line. Such a change of climate with reference to latitude must have been attended by a corresponding change of climate with reference to altitude. Three degrees of temperature correspond to about one thousand feet of altitude. If, therefore, it is found that the ancient limit of glacier action descends on the Andes, for instance, to 7,000 feet above the level of the sea under the equator, the present line of perpetual snow being at 15,000, it is safe to infer that in those days the climate was some 24° or thereabouts below its present temperature. That is, the temperature of the present snow line then prevailed at a height of 7,000 feet above the sea level, as the present average temperature of Greenland then prevailed in latitude 36°. I am as confident that we shall find these indications at about the limit I have pointed out as if I had already seen them. I would even venture to prophesy that the first moraines in the valley of the Marañon should be found where it bends eastward above Jaen.”[[9]]
Although the weather is fine, the motion of the ship continues to be so great that those of us who have not what are popularly called “sea-legs,” have much ado to keep our balance. For my own part, I am beginning to feel a personal animosity to “the trades.” I had imagined them to be soft, genial breezes wafting us gently southward; instead of which they blow dead ahead all the time, and give us no rest night or day. And yet we are very unreasonable to grumble; for never were greater comforts and conveniences provided for voyagers on the great deep than are to be found on this magnificent ship. The state-rooms large and commodious, parlor and dining-hall well ventilated, cool, and cheerful, the decks long and broad enough to give a chance for extensive “constitutionals” to everybody who can stand upright for two minutes together, the attendance punctual and admirable in every respect; in short, nothing is left to be desired except a little more stable footing.
April 10th.—A rough sea to-day, notwithstanding which we had our lecture as usual, though I must say, that, owing to the lurching of the ship, the lecturer pitched about more than was consistent with the dignity of science. Mr. Agassiz returned to the subject of embryology, urging upon his assistants the importance of collecting materials for this object as a means of obtaining an insight into the deeper relations between animals.
“Heretofore classification has been arbitrary, inasmuch as it has rested mainly upon the interpretation given to structural differences by various observers, who did not measure the character and value of these differences by any natural standard. I believe that we have a more certain guide in these matters than opinion or the individual estimate of any observer, however keen his insight into structural differences. The true principle of classification exists in Nature herself, and we have only to decipher it. If this conviction be correct, the next question is, How can we make this principle a practical one in our laboratories, an active stimulus in our investigations? Is it susceptible of positive demonstration in material facts? Is there any method to be adopted as a correct guide, if we set aside the idea of originating systems of classification of our own, and seek only to read that already written in nature? I answer, Yes. The standard is to be found in the changes animals undergo from their first formation in the egg to their adult condition.
“It would be impossible for me here and now to give you the details of this method of investigation, but I can tell you enough to illustrate my statement. Take a homely and very familiar example, that of the branch of Articulates. Naturalists divide this branch into three classes,—Insects, Crustacea, and Worms; and most of them tell you that Worms are lowest, Crustacea next in rank, and that Insects stand highest, while others have placed the Crustacea at the head of the group. We may well ask why. Why does an insect stand above a crustacean, or, vice versa, why is a grasshopper or a butterfly structurally superior to a lobster or a shrimp? And indeed there must be a difference in opinion as to the respective standing of these groups so long as their classification is allowed to remain a purely arbitrary one, based only upon interpretation of anatomical details. One man thinks the structural features of Insects superior, and places them highest; another thinks the structural features of the Crustacea highest, and places them at the head. In either case it is only a question of individual appreciation of the facts. But when we study the gradual development of the insect, and find that in its earliest stages it is worm-like, in its second, or chrysalis stage, it is crustacean-like, and only in its final completion it assumes the character of a perfect insect, we have a simple natural scale by which to estimate the comparative rank of these animals. Since we cannot suppose that there is a retrograde movement in the development of any animal, we must believe that the insect stands highest, and our classification in this instance is dictated by Nature herself. This is one of the most striking examples, but there are others quite as much so, though not as familiar. The frog, for instance, in its successive stages of development, illustrates the comparative standing of the orders composing the class to which it belongs. These orders are differently classified by various naturalists, according to their individual estimate of their structural features. But the growth of the frog, like that of the insects, gives us the true grade of the type.[[10]] There are not many groups in which this comparison has been carried out so fully as in the insects and frogs; but wherever it has been tried it is found to be a perfectly sure test. Occasional glimpses of these facts, seen disconnectedly, have done much to confirm the development theory, so greatly in vogue at present, though under a somewhat new form. Those who sustain these views have seen that there was a gradation between animals, and have inferred that it was a material connection. But when we follow it in the growth of the animals themselves, and find that, close as it is, no animal ever misses its true development, or grows to be anything but what it was meant to be, we are forced to admit that the gradation which unquestionably unites all animals is an intellectual, not a material one. It exists in the Mind which made them. As the works of a human intellect are bound together by mental kinship, so are the thoughts of the Creator spiritually united. I think that considerations like these should be an inducement for us all to collect the young of as many animals as possible on this journey. In so doing we may change the fundamental principles of classification, and confer a lasting benefit on science.
“It is very important to select the right animals for such investigations. I can conceive that a lifetime should be passed in embryological studies, and yet little be learned of the principles of classification. The embryology of the worm, for instance, would not give us the natural classification of the Articulates, because we should see only the first step of the series; we should not reach the sequence of the development. It would be like reading over and over again the first chapter of a story. The embryology of the Insects, on the contrary, would give us the whole succession of a scale on the lowest level of which the Worms remain forever. So the embryology of the frog will give us the classification of the group to which it belongs, but the embryology of the Cecilia, the lowest order in the group, will give us only the initiatory steps. In the same way the naturalist who, in studying the embryology of the reptiles, should begin with their lowest representatives, the serpents, would make a great mistake. But take the alligator, so abundant in the regions to which we are going. An alligator’s egg in the earliest condition of growth has never been opened by a naturalist. The young have been occasionally taken from the egg just before hatching, but absolutely nothing is known of their first phases of development. A complete embryology of the alligator would give us not only the natural classification of reptiles as they exist now, but might teach us something of their history from the time of their introduction upon earth to the present day. For embryology shows us not only the relations of existing animals to each other, but their relations to extinct types also. One prominent result of embryological studies has been to show that animals in the earlier stages of their growth resemble ancient representatives of the same type belonging to past geological ages. The first reptiles were introduced in the carboniferous epoch, and they were very different from those now existing. They were not numerous at that period; but later in the world’s history there was a time, justly called the ‘age of reptiles,’ when the gigantic Saurians, Plesiosaurians, and Ichthyosaurians abounded. I believe, and my conviction is drawn from my previous embryological studies, that the changes of the alligator in the egg will give us the clew to the structural relations of the Reptiles from their first creation to the present day,—will give us, in other words, their sequence in time as well as their sequence in growth. In the class of Reptiles, then, the most instructive group we can select with reference to the structural relations of the type as it now exists, and their history in past times, will be the alligator. We must therefore neglect no opportunity of collecting their eggs in as large numbers as possible.
“There are other animals in Brazil, low in their class to be sure, but yet very important to study embryologically, on account of their relation to extinct types. These are the sloths and armadillos,—animals of insignificant size in our days, but anciently represented in gigantic proportions. The Megatherium, the Mylodon, the Megalonyx, were some of these immense Mammalia. I believe that the embryonic changes of the sloths and armadillos will explain the structural relations of those huge Edentata and their connection with the present ones. South America teems with the fossil bones of these animals, which indeed penetrated into the northern half of the hemisphere as high up as Georgia and Kentucky, where their remains have been found. The living representatives of the family are also numerous in South America, and we should make it one of our chief objects to get specimens of all ages and examine them from their earliest phases upward. We must, above all, try not to be led away from the more important aims of our study by the diversity of objects. I have known many young naturalists to miss the highest success by trying to cover too much ground,—by becoming collectors rather than investigators. Bitten by the mania for amassing a great number and variety of species, such a man never returns to the general consideration of more comprehensive features. We must try to set before ourselves certain important questions, and give ourselves resolutely to the investigation of these points, even though we should sacrifice less important things more readily reached.
“Another type full of interest, from an embryological point of view, will be the Monkeys. Since some of our scientific colleagues look upon them as our ancestors, it is important that we should collect as many facts as possible concerning their growth. Of course it would be better if we could make the investigation in the land of the Orangs, Gorillas, and Chimpanzees,—the highest monkeys and the nearest to man in their development. Still even the process of growth in the South American monkey will be very instructive. Give a mathematician the initial elements of a series, and he will work out the whole; and so I believe when the laws of embryological development are better understood, naturalists will have a key to the limits of these cycles of growth, and be able to appoint them their natural boundaries even from partial data.
“Next in importance I would place the Tapirs. This is one of a family whose geological antecedents are very important and interesting. The Mastodons, the Palæotherium, the Dinotherium, and other large Mammalia of the Tertiaries, are closely related to the Tapir. The elephant, rhinoceros, and the like, are of the same family. From its structural standing next to the elephant, which is placed highest in the group, the embryology of the Tapir would give us a very complete series of changes. It would seem from some of the fossil remains of this family that the Pachyderms were formerly more nearly related to the Ruminants and Rodents than they now are. Therefore it would be well to study the embryology of the Capivari, the Paca, and the Peccary, in connection with that of the Tapir. Lastly, it will be important to learn something of the embryology of the Manatee or Sea-Cow of the Amazons. It is something like a porpoise in outline, and seems to be the modern representative of the ancient Dinotherium.”