April 17th.—Yesterday was Easter Sunday, and the day was beautiful. The services from Bishop Potter in the morning were very interesting; the more so for us on account of the God speed he gave us. Wind and weather permitting, it is the last Sunday we shall pass on board ship together. The Bishop spoke with much earnestness and sympathy of the objects of the expedition, addressing himself especially to the young men, not only with reference to their duties as connected with a scientific undertaking, but as American citizens in a foreign country at this time of war and misapprehension.
This morning we were quite entertained at meeting a number of the so-called “Catamarans,” the crazy crafts of the fishermen, who appear to be amphibious animals on this coast. Their boats consist of a few logs lashed together, over which the water breaks at every moment without apparently disturbing the occupants in the least. They fish, walk about, sit, lie down or stand, eat, drink, and sleep, to all appearance as contented and comfortable as we are in our princely steamer. Usually they go into port at nightfall, but are occasionally driven out to sea by the wind, and may sometimes be met with two hundred miles and more from the shore. To-day we have fairly come upon the South American coast. Yesterday we could catch sight occasionally of low sand banks; but this morning we have sailed past the pretty little town of Olinda, with its convent on the hill, and the larger city of Pernambuco, whose white houses come quite down to the sea-shore. Immediately in front of the town lies the reef, which runs southward along the coast for a hundred miles and more, enclosing between itself and the shore a strip of quiet waters, forming admirable anchorage for small shipping. Before Pernambuco this channel is quite deep, and directly in front of the town there is a break in the reef forming a natural gateway through which large vessels can enter. We have now left the town behind, but the shore is still in sight; a flat coast rising into low hills behind, and here and there dotted with villages and fishing-huts.
The lecture on Saturday was rather practical than scientific, on the best modes of collecting and preserving specimens, the instruments to be used, &c. To-day it was upon the classification of fishes as illustrated by embryology; the same method of study as that explained the other day and now applied to the class of fishes. “All fishes at the time when the germ becomes distinct above the yolk have a continuous fin over the whole back, around the tail, and under the abdomen. The naked reptiles, those which have no scales, such as frogs, toads, salamanders, and the like, share in this embryological feature of the fishes. From this identity of development I believe the naked reptiles to be structurally nearer to the true fishes than to the scaly reptiles. All fishes, and indeed all Vertebrates, even the highest, have, at this early period, fissures in the side of the neck. These are the first indications of gills, an organ the basis for which exists in all Vertebrates at a certain period of their life, but is fully developed and functionally active only in the lower ones, in which it acquires a special final structure; giving place to lungs in the higher ones before they reach their adult condition. From this time forward not only the class characters, but those of the family, begin to be distinguished. I will show you to-day how we may improve the classification of fishes by studying their embryology. Take, for instance, the family of Cods in its widest acceptation. It consists of several genera, among which are the Cod proper, the Cusk, and the Brotula. Naturalists may differ in their estimation of the relative rank of these genera, and even with reference to their affinity, but the embryology of the Cod seems to me to give the natural scale. In its early condition the Cod has the continuous fin of the Brotula, next the dorsal and caudal fins become distinct, as in the Cusk, and lastly the final individualization of the fins takes place, and they break up into the three dorsals and two anals of the Cod. Thus the Brotula represents the infantile condition of the Cod, and of course stands lowest, while the Cusk has its natural position between the two. There are other genera belonging to this family, as, for instance, the Lota or fresh-water Cusk and the Hake, the relative position of which may be determined by further embryological studies. I had an opportunity of observing something in the development of the Hake which throws some light on the relation of the Ophidini to the Cod family, though thus far they have been associated with the Eel. The little embryonic Hake on which I made my investigation was about an inch and a half in length; it was much more slender and elongated in proportion to its thickness than any of the family of Cods in their adult condition, and had a continuous fin all around the body. Although the structural relations of the Eels are not fully understood, some of them, at least, now united as a distinct family under the name of Ophidini, are known to be closely connected with the Cods, and this character of the Hake in its early condition would seem to show that this type of Eel is a sort of embryonic form of the Cod family.
“Another well-known family of fishes is that of the Lophioides. To this group belongs the Lophius or Goose-fish, with which the Cottoids or Sculpins, and the Blennioids, with Zoarces and Anarrhichas, the so-called Sea-cat, ought to be associated. It was my good fortune to have an opportunity of studying the development of the Lophius, and to my surprise I found that its embryonic phases included the whole series here alluded to, thus presenting another of those natural scales on which I hope all our scientific classifications will be remodelled when we obtain a better knowledge of embryology. The Lophius in its youngest stage recalls the Tænioids, being long and compressed; next it resembles the Blennioids, and growing stouter passes through a stage like Cottus, before it assumes the depressed form of Lophius. In the family of Cyprinodonts I have observed the young of Fundulus. They are destitute of ventrals, thus showing that the genus Orestias stands lowest in its family. I would allude to one other fact of this kind observed by Professor Wyman. There has been a doubt among naturalists as to the relative standing of the Skates and Sharks. On geological evidence I had placed the Skates highest, because the Sharks precede them in time; but this fact had not been established on embryological evidence. Professor Wyman has followed the embryology of the Skate through all its phases, and has found that in its earlier condition it is slender in outline, with the appearance of a diminutive shark, and that only later it assumes the broad shield-like form and long tapering tail of the skate. Were it only that they enable us to set aside all arbitrary decisions and base our classifications on the teachings of nature, these investigations would be invaluable; but their importance is increased by the consideration that we are thus gradually led to recognize the true affinities which bind all organized beings into one great system.”
April 20th.—The day after to-morrow we shall enter the Bay of Rio de Janeiro. One begins to see already that little disturbance in the regularity of sea life which precedes arrival. People are making up their letters, and rearranging their luggage; there is a slight stir pervading our small party of passengers and breaking up the even tenor of the uniform life we have been leading together for the last three weeks. It has been a delightful voyage, and yet, under the most charming circumstances, life at sea is a poor exchange for life on land, and we are all glad to be near our haven.
On Tuesday the lecture was upon the formation and growth of the egg; a sort of practical lesson in the study of embryology; yesterday, upon the importance of ascertaining, at the outset, the spawning season of the animals in Brazil, and the means to that end. “It will often be impossible for us to learn the breeding season of animals, a matter in which country people are generally very ignorant. But when we cannot obtain it from persons about us, there are some indications in the animals themselves which may serve as a guide. During my own investigations upon the development of the turtles, when I opened many thousands of eggs, I found that in these animals, at least, the appearance of the ovaries is a pretty good guide. They always contain several sets of eggs. Those which will be laid this year are the largest; those of the following year are next in size; those of two years hence still smaller, until we come to eggs so small that it is impossible to perceive any difference between their various phases of development. But we can readily tell whether there are any eggs so advanced as to be near laying, and distinguish between the brood of the year and those which are to be hatched later. When the eggs are about to be laid the whole surface is covered with ramifying blood-vessels, and the yolk is of a very clear bright yellow. Before the egg drops from the ovary this network bursts; it shrivels up and forms a little scar on the side of the ovary. Should we, therefore, on examining the ovary of a turtle, find that these scars are fresh, we may infer that the season for laying is not over; or if we find some of the eggs much larger than the rest and nearly mature, we shall know that it is about to begin. How far this will hold good with respect to alligators and other animals I do not know. I have learned to recognize these signs in the turtles from my long study of their embryology. With fishes it could hardly be possible to distinguish the different sets of eggs because they lay such numbers, and they are all so small. But if we cannot distinguish the eggs of the different years, it will be something to learn the size of their broods, which differs very greatly in different families.”
The lecture concluded with some advice as to observing and recording the metamorphoses of insects. “Though much has been written on the societies of ants and other like communities in Brazil, the accounts of different naturalists do not agree. It would be well to collect the larvæ of a great many insects, and try to raise them; but as this will be difficult and often impossible in travelling, we must at least get the nests of ants, bees, wasps, and the like, in order to ascertain all we can respecting their communities. When these are not too large it is easy to secure them by slipping a bag over them, thus taking the whole settlement captive. It may then be preserved by dipping into alcohol, and examined at leisure, so as to ascertain the number and nature of the individuals contained in it, and learn something at least of their habits. Nor let us neglect the domestic establishments of spiders. There is an immense variety of spiders in South America, and a great difference in their webs. It would be well to preserve these on sheets of paper, to make drawings of them, and examine their threads microscopically.”
April 21st.—Yesterday Mr. Agassiz gave his closing lecture, knowing that to-day all would be occupied with preparations for landing. He gave a little history of Steenstrup and Sars, and showed the influence their embryological investigations have had in reforming classification, and also their direct bearing upon the question of the origin of species. To these investigators science owes the discovery of the so-called “alternate generations,” in which the Hydroid, either by budding or by the breaking up of its own body, gives rise to numerous jelly-fishes; these lay eggs which produce Hydroids again, and the Hydroids renew the process as before.[[14]]
“These results are but recently added to the annals of science, and are not yet very extensively known in the community; but when the facts are more fully understood, they cannot fail to affect the fundamental principles of zoölogy. I have been astonished to see how little weight Darwin himself gives to this series of transformations; he hardly alludes to it, and yet it has a very direct bearing on his theory, since it shows that, however great the divergence from the starting-point in any process of development, it ever returns to the road of its normal destiny; the cycle may be wide, but the boundaries are as impassable as if it were narrower. However these processes of development may approach, or even cross each other, they never end in making any living being different from the one which gave it birth, though in reaching that point it may pass through phases resembling other animals.
“In considering these questions we should remember how slight are most of those specific differences, the origin of which gives rise to so much controversy, in comparison with the cycle of changes undergone by every individual in the course of its development. There are numerous genera, including many very closely allied species, distinguished by differences which, were it not for the fact that they have remained unchanged and invariable through ages, might be termed insignificant. Such, for instance, are the various species of corals found in the everglades of Florida, where they lived and died ages ago, and had the identical specific differences by which we distinguish their successors in the present Florida reefs. The whole science of zoölogy in its present condition is based upon the fact that these slight differences are maintained generation after generation. And yet every individual on such a coral stock,—and the same is true of any individual in any class whatsoever of the whole animal kingdom, whether Radiate, Mollusk, Articulate, or Vertebrate,—before reaching its adult condition and assuming the permanent characters which distinguish it from other species, and have never been known to vary, passes in a comparatively short period through an extraordinary transformation, the successive phases of which differ far more from each other than do the adult species. In other words, the same individual differs more from himself in successive stages of his growth than he does in his adult condition from kindred species of the same genus. The conclusion seems inevitable, that, if the slight differences which distinguish species were not inherent, and if the phases through which every individual has to pass were not the appointed means to reach that end, themselves invariable, there would be ever-recurring deviations from the normal types. Every naturalist knows that this is not the case. All the deviations known to us are monstrosities, and the occurrence of these, under disturbing influences, are to my mind only additional evidence of the fixity of species. The extreme deviations obtained in domesticity are secured, as is well known, at the expense of the typical characters, and end usually in the production of sterile individuals. All such facts seem to show that so-called varieties or breeds, far from indicating the beginning of new types, or the initiating of incipient species, only point out the range of flexibility in types which in their essence are invariable.