Standing in the neighborhood of Whitehall, near Lake George, one may look along such a sea-shore, and see it stretching westward and sloping gently southward as far as the eye can reach. It must have had a very gradual slope, and the waters must have been very shallow; for at that time no great mountains had been uplifted, and deep oceans are always the concomitants of lofty heights. We do not, however, judge of this by inference merely; we have an evidence of the shallowness of the sea in those days in the character of the shells found in the Silurian deposits, which shows that they belonged in shoal waters.
Indeed, the fossil remains of all times tell us almost as much of the physical condition of the world at different epochs as they do of its animal and vegetable population. When Robinson Crusoe first caught sight of the footprint on the sand, he saw in it more than the mere footprint, for it spoke to him of the presence of men on his desert island. We walk on the old geological shores, like Crusoe along his beach, and the footprints we find there tell us, too, more than we actually see in them. The crust of our earth is a great cemetery where the rocks are tombstones on which the buried dead have written their own epitaphs. They tell us not only who they were and when and where they lived, but much also of the circumstances under which they lived. We ascertain the prevalence of certain physical conditions at special epochs by the presence of animals and plants whose existence and maintenance required such a state of things, more than by any positive knowledge respecting it. Where we find the remains of quadrupeds corresponding to our ruminating animals, we infer not only land, but grassy meadows also, and an extensive vegetation; where we find none but marine animals, we know the ocean must have covered the earth; the remains of large reptiles, representing, though in gigantic size, the half aquatic, half terrestrial reptiles of our own period, indicate to us the existence of spreading marshes still soaked by the retreating waters; while the traces of such animals as live now in sand and shoal waters, or in mud, speak to us of shelving sandy beaches and of mud-flats. The eye of the Trilobite tells us that the sun shone on the old beach where he lived; for there is nothing in Nature without a purpose, and when so complicated an organ was made to receive the light, there must have been light to enter it. The immense vegetable deposits in the Carboniferous period announce the introduction of an extensive terrestrial vegetation; and the impressions left by the wood and leaves of the trees show that these first forests must have grown in a damp soil and a moist atmosphere. In short, all the remains of animals and plants hidden in the rocks have something to tell of the climatic conditions and the general circumstances under which they lived, and the study of fossils is to the naturalist a thermometer by which he reads the variations of temperature in past times, a plummet by which he sounds the depths of the ancient oceans,—a register, in fact, of all the important physical changes the earth has undergone.
But although the animals of the early geological deposits indicate shallow seas by their similarity to our shoal-water animals, it must not be supposed that they are by any means the same. On the contrary, the old shells, crustacea, corals, etc., represent types which have existed in all times with the same essential structural elements, but under different specific forms in the several geological periods. And here it may not be amiss to say something of what are called by naturalists representative types.
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. There 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 the 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 faunae. 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 or zoological provinces called faunae 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 clue 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 faunae under various zones, their zoological 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 tangled 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 sea-shores, 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.
What we do find—and these, as I shall endeavor to show my readers, in such profusion that it would seem as if God, in the joy of creation, had compensated Himself for a less variety of forms in the greater richness of the early types—is an immense number of beings belonging to the four primary divisions of the Animal Kingdom, but only to those classes whose representatives are marine, whose home then, as now, was either in the sea or along its shores. In other words, the first organic creation expressed in its totality the structural conception since carried out in such wonderful variety of details, and purposely limited then, because the world, which was to be the home of the higher animals, was not yet made ready to receive them.
I am fully aware that the intimate relations between the organic and physical world are interpreted by many as indicating the absence, rather than the presence, of an intelligent Creator. They argue, that the dependence of animals on material laws gives us the clue to their origin as well as to their maintenance. Were this influence as absolute and unvarying as the purely mechanical action of physical circumstances must necessarily be, this inference might have some pretence to logical probability,—though it seems to me unnecessary, under any circumstances, to resort to climatic influences or the action of any physical laws to explain the thoughtful distribution of the organic and inorganic world, so evidently intended to secure for all beings what best suits their nature and their needs. But the truth is, that, while these harmonious relations underlie the whole creation in such a manner as to indicate a great central plan, of which all things are a part, there is at the same time a freedom, an arbitrary element in the mode of carrying it out, which seems to point to the exercise of an individual will; for, side by side with facts, apparently the direct result of physical laws, are other facts, the nature of which shows a complete independence of external influences.