C.D. Warner, et al., comp.
The Library of the World’s Best Literature. An Anthology in Thirty Volumes. 1917.
The Silurian Beach
By Louis Agassiz (1807–1873)
W
To me it seems, that to look on the first land that was ever lifted above the wasted waters, to follow the shore where the earliest animals and plants were created when the thought of God first expressed itself in organic forms, to hold in one’s hand a bit of stone from an old sea-beach, hardened into rock thousands of centuries ago, and studded with the beings that once crept upon its surface or were stranded there by some retreating wave, is even of deeper interest to men than the relics of their own race, for these things tell more directly of the thoughts and creative acts of God….
The statement that different sets of animals and plants have characterized the successive epochs is often understood as indicating a difference of another kind than that which distinguishes animals now living in different parts of the world. This is a mistake. They are so-called representative types all over the globe, united to each other by structural relations and separated by specific differences of the same kind as those that unite and separate animals of different geological periods. Take, for instance, mud-flats or sandy shores in the same latitudes of Europe and America: we find living on each, animals of the same structural character and of the same general appearance, but with certain specific differences, as of color, size, external appendages, etc. They represent each other on the two continents. The American wolves, foxes, bears, rabbits, are not the same as the European, but those of one continent are as true to their respective types as those of the other; under a somewhat different aspect they represent the same groups of animals. In certain latitudes, or under conditions of nearer proximity, these differences may be less marked. It is well known that there is a great monotony of type, not only among animals and plants but in the human races also, throughout the Arctic regions; and some animals characteristic of the high North reappear under such identical forms in the neighborhood of the snow-fields in lofty mountains, that to trace the difference between the ptarmigans, rabbits, and other gnawing animals of the Alps, for instance, and those of the Arctics, is among the most difficult problems of modern science.
And so is it also with the animated world of past ages: in similar deposits of sand, mud, or lime, in adjoining regions of the same geological age, identical remains of animals and plants may be found; while at greater distances, but under similar circumstances, representative species may occur. In very remote regions, however, whether the circumstances be similar or dissimilar, the general aspect of the organic world differs greatly, remoteness in space being thus in some measure an indication of the degree of affinity between different faunæ. In deposits of different geological periods immediately following each other, we sometimes find remains of animals and plants so closely allied to those of earlier or later periods that at first sight the specific differences are hardly discernible. The difficulty of solving these questions, and of appreciating correctly the differences and similarities between such closely allied organisms, explains the antagonistic views of many naturalists respecting the range of existence of animals, during longer or shorter geological periods; and the superficial way in which discussions concerning the transition of species are carried on, is mainly owing to an ignorance of the conditions above alluded to. My own personal observation and experience in these matters have led me to the conviction that every geological period has had its own representatives, and that no single species has been repeated in successive ages.
The laws regulating the geographical distribution of animals, and their combination into distinct zoölogical provinces called faunæ, with definite limits, are very imperfectly understood as yet; but so closely are all things linked together from the beginning till to-day, that I am convinced we shall never find the clew to their meaning till we carry on our investigations in the past and the present simultaneously. The same principle according to which animal and vegetable life is distributed over the surface of the earth now, prevailed in the earliest geological periods. The geological deposits of all times have had their characteristic faunæ under various zones, their zoölogical provinces presenting special combinations of animal and vegetable life over certain regions, and their representative types reproducing in different countries, but under similar latitudes, the same groups with specific differences.
Of course, the nearer we approach the beginning of organic life, the less marked do we find the differences to be; and for a very obvious reason. The inequalities of the earth’s surface, her mountain-barriers protecting whole continents from the Arctic winds, her open plains exposing others to the full force of the polar blasts, her snug valleys and her lofty heights, her table-lands and rolling prairies, her river-systems and her dry deserts, her cold ocean-currents pouring down from the high North on some of her shores, while warm ones from tropical seas carry their softer influence to others,—in short, all the contrasts in the external configuration of the globe, with the physical conditions attendant upon them, are naturally accompanied by a corresponding variety in animal and vegetable life.
But in the Silurian age, when there were no elevations higher than the Canadian hills, when water covered the face of the earth with the exception of a few isolated portions lifted above the almost universal ocean, how monotonous must have been the conditions of life! And what should we expect to find on those first shores? If we are walking on a sea-beach to-day, we do not look for animals that haunt the forests or roam over the open plains, or for those that live in sheltered valleys or in inland regions or on mountain-heights. We look for Shells, for Mussels and Barnacles, for Crabs, for Shrimps, for Marine Worms, for Star-Fishes and Sea-Urchins, and we may find here and there a fish stranded on the sand or strangled in the sea-weed. Let us remember, then, that in the Silurian period the world, so far as it was raised above the ocean, was a beach; and let us seek there for such creatures as God has made to live on seashores, and not belittle the Creative work, or say that He first scattered the seeds of life in meagre or stinted measure, because we do not find air-breathing animals when there was no fitting atmosphere to feed their lungs, insects with no terrestrial plants to live upon, reptiles without marshes, birds without trees, cattle without grass,—all things, in short, without the essential conditions for their existence….
I have spoken of the Silurian beach as if there were but one, not only because I wished to limit my sketch, and to attempt at least to give it the vividness of a special locality, but also because a single such shore will give us as good an idea of the characteristic fauna of the time as if we drew our material from a wider range. There are, however, a great number of parallel ridges belonging to the Silurian and Devonian periods, running from east to west, not only through the State of New York, but far beyond, through the States of Michigan and Wisconsin into Minnesota; one may follow nine or ten such successive shores in unbroken lines, from the neighborhood of Lake Champlain to the Far West. They have all the irregularities of modern seashores, running up to form little bays here, and jutting out in promontories there….
Although the early geological periods are more legible in North America, because they are exposed over such extensive tracts of land, yet they have been studied in many other parts of the globe. In Norway, in Germany, in France, in Russia, in Siberia, in Kamchatka, in parts of South America,—in short, wherever the civilization of the white race has extended, Silurian deposits have been observed, and everywhere they bear the same testimony to a profuse and varied creation. The earth was teeming then with life as now; and in whatever corner of its surface the geologist finds the old strata, they hold a dead fauna as numerous as that which lives and moves above it. Nor do we find that there was any gradual increase or decrease of any organic forms at the beginning and close of the successive periods. On the contrary, the opening scenes of every chapter in the world’s history have been crowded with life, and its last leaves as full and varied as its first.