Louis Agassiz’s “Introduction” opens his monumental monograph on the fossil fishes of the Old Red Sandstone, written in French, and it is less a preface than a manifesto: a working naturalist steps back from the descriptive grind of species and plates to argue that the fossil record rewrites the rules of zoology itself. The form is expository—a sustained scientific argument built from comparative anatomy, stratigraphy, and embryology—and its subject is nothing less than the order of creation. Agassiz writes from the vantage point of a quarter-century of paleontological discovery, and he wants his readers to see what those discoveries have quietly made untenable.
The Case Against the Chain of Being
Agassiz begins by surveying the zoological systems in fashion. Most treatises arrange animals as a continuous series running from zoophytes up through radiates, mollusks, and articulates to man, shuffling mollusks and articulates back and forth according to each author’s sense of superiority. Others allow a gradation from invertebrates to vertebrates but split it into two parallel branches that converge at the vertebrate type; still others admit multiple series, parallel or diverging. A final camp treats the great divisions as equivalent groups that cannot be ranked at all, each a mode of existence equally perfect in its own sphere—a view in which nature contains no gradation whatsoever.
None of these schemes survives contact with the fossils, Agassiz argues. The first capital fact is the one a quarter-century of research has established: the animals now living are only a feeble fraction of those that once peopled the globe. To build a classification on living species alone is therefore arbitrary, because extinct types appear and disappear at determined epochs. As whole orders of vanished creatures come to light, the need for a more complete method grows urgent. Systems that treat the animal kingdom as a simultaneous whole falsify the primitive relations and the chronological order of creation. Before classifying organized beings, one must first know when they appeared; the epoch of appearance has become as essential a basis for classification as organization itself.
Reading the Rocks as a Sequence of Creations
From this principle Agassiz draws a program. To know animals philosophically, we must determine the state of the kingdom at its first appearance, trace the organic changes at each succeeding epoch, and fix the geological limits of those changes. Geologists, he notes approvingly, have never worked harder at dating terrains and tightening the boundaries of formations, and their labors have subdivided the accepted epochs ever more finely. The result is that paleontologists increasingly accept a plurality of distinct, independent creations. Agassiz predicts that the broad labels—primary, secondary, tertiary; paleozoic, triassic, jurassic, cretaceous—will give way to named creations: Cambrian, Silurian, Devonian, coal, Permian, each an assemblage of life comparable in importance to our own. Recent discoveries in distant countries confirm the forecast: Lund’s fossil bones from Brazil, Falconer and Cautley’s from the sub-Himalayan hills. Everywhere, within narrow vertical and horizontal limits, investigators find faunal assemblages as rich as the densest living faunas of comparable geographic extent.
The Old Red Sandstone fishes, he hopes, will supply a new argument for the theory. He announces his method: a rapid review of how the principal types of animal life developed at the chief epochs of their metamorphoses, then a demonstration of how those types combined across the series of times. He will not build a complete system; he will highlight the immediate consequences of the best-studied zoological and geological facts. The accord between zoological affinity and geological distribution is so striking that he states it as fact: a classification that is not at the same time an expression of the succession of families in time cannot express real affinities. Modern paleontology has repeatedly confirmed the happiest groupings naturalists proposed, often without knowing it. Knowledge of fossils can now guide zoological method, and anatomy can date a deposit even without identical species. A further harmony with the laws of geographic distribution he foresees but sets aside, since the facts are not yet well enough known.
Mollusks, Cephalopods, and the Order of Succession
The survey of the classes follows. Gastropods are conceded to be poorly studied in the older terrains, but one pattern holds: of the two shell types in the present creation, the one with an entire opening, without canal or notch for a respiratory tube, is the oldest, found alone in Paleozoic and older Secondary rocks, while the siphon-bearing type appears only from the lias onward and grows ever more dominant into the Tertiary and the present. Curiously, these ancient gastropods resemble terrestrial and fluviatile shells more than marine ones—a parallel to what Agassiz long ago signaled for Secondary fossil fishes, which, though belonging to extinct genera, resemble certain freshwater fishes of today more than any marine fish.
Cephalopods are among the best-studied classes, thanks to the monographs of Ferussac, d’Orbigny, de Buch, Münster, Voltz, Owen, and Valenciennes. The ammonites and nautili are the oldest types, roughly contemporaneous in their development—a possible new proof of their value as zoological divisions—but they are not of equal weight. The ammonite family, more numerous and varied in older epochs, dies out earlier, never passing the Cretaceous. D’Orbigny’s many chalk genera, appearing with astonishing diversity just as the family is about to vanish, give a true image of the convulsive movements of the ammonite-generating thought in its agony. The seiches and poulpes, the class’s third and highest type, do not go back beyond the lias, where the belemnites and their kin were precursors of our cuttlefish and squid.
The Articulates and the Embryo’s Lesson
The articulate branch counts three classes—crustaceans, insects, and worms—and Agassiz insists on reuniting what others split. Cirripedes cannot be separated from crustaceans, whose organization and development they share; the lernees and rotifers also belong there. Arachnids and myriapods are true insects, or link to winged insects by types so closely united that separation is impossible. As for worms, annelids, turbellarians, and helminths cannot stand as separate classes; their common characters and the analogy of their embryogenic development are too striking. Intestinal worms have no place among the radiates, nor do the infusoria, which attach in great majority to crustaceans through the rotifers.
Worms and insects have left too few traces to judge their ancient role; they await their monographers. Crustaceans, by contrast, are known well enough to trace their development from the remotest times. Trilobites, the oldest type, appear in the most varied and strange forms from their first appearance in the oldest Paleozoic terrains, yet they do not pass the coal epoch, where gigantic entomostracans replace them as forerunners of the macrurans. The macrurans reign from the Triassic into the present; the brachyurans are essentially Tertiary, as are the cirripedes, which everywhere accompany them—and a monograph of living and fossil cirripedes is a pressing desideratum for both zoology and paleontology. Parasitic, soft, vermiform crustaceans belong exclusively to the present creation. The pattern that emerges is decisive: the types whose affinities are best studied—trilobites, macrurans, brachyurans—also succeed one another in the rocks in the order of their organic gradation, and their forms bear an intimate analogy to the phases of crustacean embryonic development described by Rathke and Erdl.
The Reign of Fishes
The Old Red Sandstone’s ichthyological fauna is so extraordinary that the least debris of its creatures strikes the naturalist. No other formation gathers fishes so far from anything alive today, and none required so many years before its types could be classified. The difficulties were peculiar because they had to be solved without terms of comparison: the first complete ichthyological fauna is the Old Red’s own. Silurian rocks hold a few fish fragments, too rare to help; only with the Devonian did fishes acquire real importance. And here fishes stand alone as the sole representatives of the vertebrate branch—not only in the Old Red but in the coal measures too—so the epoch may rightly be called the reign of fishes. Recent research has multiplied the known species tenfold, and further discoveries in England and Russia will fall within the same law: fishes only, with no reptiles or mammals in these layers. A recent author claimed to find bones of all vertebrate classes there, but his turtles, lizards, crocodiles, and pachyderms were false determinations, each reassigned to the lowest vertebrate class; Agassiz’s own treatment of the Devonian families dismantled the scaffolding, leaving it proved that every known vertebrate fragment older than the Zechstein belongs to fishes.
Against this background Agassiz states his law: successive creations passed through phases of development analogous to those of the embryo, and similar to the gradations visible in the ascending series of the present creation. The fish embryo, the class of living fishes across its families, and the fish type across its planetary history all follow the same creative thought as a guiding thread. That the Old Red fishes represent the embryonic age of the reign of fishes is the motive behind the whole monograph.
Five Families, One Embryonic Grade
Five families are enumerated: the Cephalaspides, the Acanthodians, the dipterian Sauroids, the Coelacanths, and the Plagiostomes—the first four ganoids, the last placoids. The first observation is that in the numerous species no trace of vertebral bodies has been found, only in a few cases apophyses to protect the spinal cord and great vessels. Had vertebral bodies existed, they would lie among the abundant skeleton debris—on the Coccosteus plates of the Orkney, where tails are preserved with their spinous apophyses, interapophysary ossicles, and fin rays. None exist; the apophyses rest on an undivided, continuous axis. This incomplete development of the trunk’s bony system is found in all embryos, especially fish embryos, and in the lowest rung of the fish class, the cyclostomes. The vertebral series is replaced by the gelatinous dorsal cord; only later do apophyses and vertebrae develop. In the Branchiostoma there is only a cord; in cyclostomes the apophyses begin; in plagiostomes the vertebral bodies. The Old Red fishes remained at a wholly embryonic degree—a cord and apophyses, but no vertebral bodies.
This almost necessarily determines a second trait: an incomplete skull. The exterior bones—jaws, thoracic belt, opercular and branchiostegal bones, the plates on the skull’s roof—are well formed, vigorous, evidently bony. But everything suggests the internal cranial box was not yet consolidated, remaining cartilaginous. Embryos show the same structure, protecting plates developing in isolation over a still-cartilaginous box, and so does the sturgeon, with which the Old Red fishes’ skull skeleton can best be compared. The enameled plates covering the sturgeon’s head, continuing those of neck and flanks, are cutaneous bones developed in place of ordinary bones—exactly the arrangement in the Cephalaspides. It would be wasted effort to seek frontals and parietals in the cephalic plates of a Coccosteus or Pterichthys; in their place are carapaces, often strangely composed, forming cranial coverings as complete as any ordinary fish’s.
Short-Lived Bizarre Types and the Corsairs of the First Oceans
The Acanthodians occur only in the Old Red and the coal measures; later strata show no trace. Their body form is familiar, but their covering is unique: tiny rhomboidal scales, barely visible, giving the skin a shagreen appearance formed of elements entirely different from the placoid shagreen. Agassiz draws a general lesson—bizarre types that depart most from the normal last only one or two epochs, ending without replacements. So it is with the Cephalaspides and the Acanthodians alike; in the fusiform ganoids of later epochs there are neither shagreen scales nor the great spine-like prickles in the fins. The type dies out entirely with the coal.
The Coelacanths alone among Old Red ganoids have a prolonged history, continuing into the chalk, where they end with Macropoma. Agassiz has already shown how hard the family is to limit sharply and how likely to divide; setting that aside, the Old Red is certainly its heyday, diminishing in all directions until extinction. Graphically it is a cone with a broad base—the Holoptychius, Phyllolepis, Glyptolepis, Platygnathes, Dendrodus, Lamnodus, Cricodus, Asterolepis, Bothriolepis, Psammosteus of the Devonian—and Macropoma at the summit. Some Russian beds are true breccias composed almost solely of Asterolepis or Bothriolepis scales and plates. Pterichthys are so abundant in the Lethen-Bar geodes that cartloads are gathered, which is no wonder for small fish probably living in troops in the mud and feeding on shell-less mollusks and worms. But Bothriolepis and Asterolepis were very large, eminently rapacious fish, judging by their dentition, and it is astonishing that such voracious species—whose analogues today are scattered—should be assembled in such numbers in certain localities.
Within these Coelacanths several distinct types already appear: the Glyptolepis, whose double fins approach the dipterian Sauroids so closely that one might believe in a parallelism of the two families; and the Asterolepis, Bothriolepis, and Psammosteus, furnished with broad cutaneous plates and approaching by their dentition the true Coelacanth type of Holoptychius and Platygnathes. These two groups were the absolute sovereigns of their seas; gigantic bodies and sharp cutting teeth gave them uncontested superiority. In the coal, true Sauroids of equally remarkable size—Megalichthys among them—join these corsairs of the first oceans, and in later formations the Sauroids gain the upper hand. The dentition is remarkable: all these fish except the Glyptolepis have sharp, isolated teeth formed of folded dentine, and nowhere else in the animal kingdom does that folding go as far—witness Dendrodus and Lamnodus.
The Old Red placoids are not yet well enough known to fix their relations with later formations. What struck Agassiz most is the smallness of the ichthyodorulithes compared with those of the coal and the lias, and the rarity of teeth relative to the abundance of spiny rays—the inverse of the Cretaceous, the Tertiary, and living species. He concludes that in the first times of life’s development it was less the placoids than certain ganoids, the Coelacanths and Sauroids especially, that made the terror of the seas and traversed them as masters, as the sharks do today under all latitudes.
What the Argument Means
The essay’s central claim is methodological before it is theological: classification must be historical. A system of animals that ignores the order of their appearance in time is, for Agassiz, simply false to nature, and the fossil record—not the living fauna—supplies the missing dimension. His second claim is the law of recapitulation in a bold, planetary form: the embryo, the class, and the history of the type on earth run through analogous phases, so that the Old Red fishes, arrested at the embryonic grade of cord and apophyses, can be read as the infancy of the fish type itself. The third is a portrait of deep time as a series of independent creations, each with its own assemblage, its own duration, and often its own brief freaks—types too bizarre to last. Whether one accepts the law or not, the essay shows a working scientist using anatomy and stratigraphy together as a single instrument, and staking his monograph’s importance on the wager that they cannot be separated.
