Louis Agassiz: His Life and Correspondence

by Louis Agassiz

You may hardly feel an interest in my zoological observations, but I am sure you will be glad to learn that we had the best opportunity of carefully examining most of the animals known to inhabit the Gulf weed, and some also which I did not know to occur among them. The most interesting discovery of our voyage thus far, however, is that of a nest built by a fish, and floating on the broad ocean with its living freight. On the 13th, Mr. Mansfield, one of our officers, brought me a ball of Gulf weed which he had just picked up, and which excited my curiosity to the utmost. It was a round mass of Sargassum about the size of two fists. The bulk of the ball was made up of closely packed branches and leaves, held together by fine threads, running through them in every direction, while other branches hung more loosely from the margin. Placed in a large bowl of water it became apparent that the loose branches served to keep the central mass floating, cradle-like, between them. The elastic threads, which held the ball of Gulf weed together, were beaded at intervals, sometimes two or three beads close together, or a bunch of them hanging from the same cluster of threads, or occasionally scattered at a greater distance from each other. Nowhere was there much regularity in the distribution of the beads. They were scattered pretty uniformly throughout the whole ball of seaweed, and were themselves about the size of an ordinary pin's head. Evidently we had before us a nest of the most curious kind, full of eggs. What animal could have built this singular nest? It did not take long to ascertain the class to which it belonged. A common pocket lens revealed at once two large eyes on the side of the head, and a tail bent over the back of the body, as in the embryo of ordinary fishes shortly before the period of hatching. The many empty egg cases in the nest gave promise of an early opportunity of seeing some embryos, freeing themselves from their envelope. Meanwhile a number of these eggs containing live embryos were cut out of the nest and placed in separate glass jars, in order to multiply the chances of preserving them; while the nest as a whole was secured in alcohol, as a memorial of our discovery.

The next day I found two embryos in my glass jars; they moved occasionally in jerks, and then rested a long time motionless on the bottom of the jar. On the third day I had over a dozen of these young fishes, the oldest beginning to be more active. I need not relate in detail the evidence I soon obtained that these embryos were actually fishes. . .But what kind of fish was it? At about the time of hatching, the fins differ too much from those of the adult, and the general form has too few peculiarities, to give any clew to this problem. I could only suppose it would prove to be one of the pelagic species of the Atlantic. In former years I had made a careful study of the pigment cells of the skin in a variety of young fishes, and I now resorted to this method to identify my embryos. Happily we had on board several pelagic fishes alive. The very first comparison I made gave the desired result. The pigment cell of a young Chironectes pictus proved identical with those of our little embryos. It thus stands, as a well authenticated fact, that the common pelagic Chironectes of the Atlantic, named Ch. pictus by Cuvier, builds a nest for its eggs in which the progeny is wrapped up with the materials of which the nest itself is composed; and as these materials consist of the living Gulf weed, the fish cradle, rocking upon the deep ocean, is carried along as in an arbor, which affords protection and afterwards food also, to its living freight. This marvelous story acquires additional interest, when we consider the characteristic peculiarities of the genus Chironectes. As its name indicates, it has fin-like hands; that is to say, the pectoral fins are supported by a kind of long wrist-like appendage, and the rays of the ventrals are not unlike rude fingers. With these limbs these fishes have long been known to attach themselves to sea-weeds, and rather to walk than to swim in their natural element. But now that we know their mode of reproduction, it may fairly be asked if the most important use of their peculiarly constructed fins is not the building of their nest?. . .There thus remains one closing chapter to the story. May some naturalist, becalmed among the Gulf weed, have the good fortune to witness the process by which the nest is built. . .

This whole investigation was of the greatest interest to Agassiz, and, coming so early in the voyage, seemed a pleasant promise of its farther opportunities. The whole ship's company soon shared his enthusiasm, and the very sailors gathered about him in the intervals of their work, or hung on the outskirts of the scientific circle. A pause of a few days was made at one or two of the West Indian islands, at St. Thomas and Barbados. At the latter, the first cast of the large dredge was made on a ledge of shoals in a depth of eighty fathoms, and, among countless other things, a number of stemmed crinoids and comatulae were brought up. An ardent student of the early fossil echinoderms, it was a great pleasure to Agassiz to gather their fresh and living representatives. It was like turning a leaf of the past and finding the subtle thread which connects it with the present.

TO PROFESSOR PEIRCE.

PERNAMBUCO, January 16, 1872.

MY DEAR PEIRCE,

I should have written to you from Barbados, but the day before we left the island was favorable for dredging, and our success in that line was so unexpectedly great, that I could not get away from the specimens, and made the most of them for study while I had the chance. We made only four hauls, in between seventy-five and one hundred and twenty fathoms. But what hauls! Enough to occupy half a dozen competent zoologists for a whole year, if the specimens could be kept fresh for that length of time. The first haul brought up a Chemidium-like sponge; the next gave us a crinoid, very much like the Rhizocrinus lofotensis, but probably different; the third, a living Pleurotomaria; the fourth, a new genus of Spatangoids, etc., etc., not to speak of the small fry. We had the crinoid alive for ten or twelve hours. When contracted, the pinnules are pressed against the arms, and the arms themselves shut against one another, so that the whole looks like a swash made up of a few long, coarse twines. When the animal opens, the arms at first separate without bending outside, so that the whole looks like an inverted pentapod; but gradually the tips of the arms bend outward as the arms diverge more and more, and when fully expanded the crown has the appearance of a lily of the L. martagon type, in which each petal is curved upon itself, the pinnules of the arms spreading laterally more and more, as the crown is more fully open. I have not been able to detect any motion in the stem traceable to contraction, though there is no stiffness in its bearing. When disturbed, the pinnules of the arms first contract, the arms straighten themselves out, and the whole gradually and slowly closes up. It was a very impressive sight for me to watch the movements of the creature, for it not only told of its own ways, but at the same time afforded a glimpse into the countless ages of the past, when these crinoids, so rare and so rarely seen nowadays, formed a prominent feature of the animal kingdom. I could see, without great effort of the imagination, the shoal of Lockport teeming with the many genera of crinoids which the geologists of New York have rescued from that prolific Silurian deposit, or recall the formations of my native country, in the hill-sides of which also, among fossils indicating shoal water deposits, other crinoids abound, resembling still more closely those we find in these waters. The close affinities of Rhizocrinus with Apiocrinoids are further exemplified by the fact that when the animal dies, it casts off its arms, like Apiocrinus, the head of which is generally found without arms. And now the question may be asked, what is the meaning of the occurrence of these animals in deep waters at the present day, when, in former ages, similar types inhabited shallow seas? Of the fact there can be no doubt, for it is not difficult to adduce satisfactory evidence of the shoal-like character of the Silurian deposits of the State of New York; their horizontal position, combined with the gradual recession of the higher beds in a southerly direction, leaves no doubt upon this point; and in the case of the jurassic formation alluded to above, the combination of the crinoids with fossils common upon coral reefs, and their presence in atolls of that period, are satisfactory proofs of my assertion. What does it mean, then, when we find the Pentacrinus and Rhizocrinus of the West Indies in deep water only? It seems to me that there is but one explanation of the fact, namely, that in the progress of the earth's growth, we must look for such a displacement of the conditions favorable to the maintenance of certain lower types, as may recall most fully the adaptations of former ages. It was in this sense I alluded, in my first letter to you, to the probability of our finding in deeper water representatives of earlier geological types; and if my explanation is correct, my anticipation is also fully sustained. But do the deeper waters of the present constitution of our globe really approximate the conditions for the development of animal life, which existed in the shallower seas of past geological ages? I think they do, or at least I believe they approach it as nearly as anything can in the present order of things upon earth; for the depths of the ocean alone can place animals under a pressure corresponding to that caused by the heavy atmosphere of earlier periods. But, of course, such high pressure as animals meet in great depths cannot be a favorable condition for the development of life; hence the predominance of lower forms in the deep sea. The rapid diminution of light with the increasing depth, and the small amount of free oxygen in these waters under greater and greater pressure, not to speak of other limitations arising from the greater uniformity of the conditions of existence, the reduced amount and less variety of nutritive substances, etc., etc., are so many causes acting in the same direction and with similar results. For all these reasons, I have always expected to find that the animals living in great depths would prove to be of a standing, in the scale of structural complications, inferior to those found in shoal waters or near shore; and the correlation elsewhere pointed out between the standing of animals and their order of succession in geological times (see "Essay on Classification ") justifies another form of expression of these facts, namely, that in deeper waters we should expect to find representatives of earlier geological periods. There is in all this nothing which warrants the conclusion that any of the animals now living are lineal descendants of those of earlier ages; nor does their similarity to those of earlier periods justify the statement that the cretaceous formation is still extant. It would be just as true to nature to say that the tertiaries are continued in the tropics, on account of the similarity of the miocene mammalia to those of the torrid zone.

We have another case in the Pleurotomaria. It is not long since it has been made known that the genus Pleurotomaria is not altogether extinct, a single specimen having been discovered about ten years ago in the West Indies. Even Pictet, in the second edition of his Paleontology, still considers Pleurotomaria as extinct, and as belonging to the fossiliferous formations which extend from the Silurian period to the Tertiary. Of the living species found at Marie Galante, nothing is known except the specific characteristics of the shell. We dredged it in one hundred and twenty fathoms, on the west side of Barbados, alive, and kept it alive for twenty-four hours, during which time the animal expanded and showed its remarkable peculiarities. It is unquestionably the type of a distinct family, entirely different from the other Mollusks with which it has been hitherto associated. Mr. Blake has made fine colored drawings of it, which may be published at some future time. . .The family of the Pleurotomariae numbers between four and five hundred fossil species, beginning in the Silurian deposits, but especially numerous in the carboniferous and jurassic formations.

The sponges afford another interesting case. When the first number of the great work of Goldfuss, on the fossils of Germany, made its appearance, about half a century ago, the most novel types it made known were several genera of sponges from the jurassic and cretaceous beds, described under the names of Siphonia, Chemidium, and Scyphia. Nothing of the kind has been known among the living to this day; and yet, the first haul of the dredge near Barbados gave us a Chemidium, or, at least, a sponge so much like the fossil Chemidium, that it must remain for future comparisons to determine whether there are any generic differences between our living sponge and the fossil. The next day brought us a genuine Siphonia, another genus thus far only known from the jurassic beds; and it is worth recording, that I noticed in the collection of Governor Rawson another sponge,—brought to him by a fisherman who had caught it on his line, on the coast of Barbados,—which belongs to the genus Scyphia. Thus the three characteristic genera of sponges from the secondary formation, till now supposed to be extinct, are all three represented in the deep waters of the West Indies. . .

Another family of organized beings offers a similar testimony to that already alluded to. If there is a type of Echinoderms characteristic of a geological period, it is the genus Micraster of the cretaceous formation, in its original circumscription. No species of this genus is known to have existed during the Tertiary era, and no living species has as yet been made known. You may therefore imagine my surprise when the dredge first yielded three specimens of a small species of that particular group of the genus, which is most extensively represented in the upper cretaceous beds.