No one can doubt that the earth’s crust, so far as it has been deciphered by man, presents us with a record, imperfect though it be, of the past. Whether, however, the known and admitted imperfections of its records, geological and palæontological, are sufficiently trustworthy to account satisfactorily for the lack of direct evidence recognizable in some modern hypotheses, may be a matter of individual opinion, but there can be little doubt that they are sufficiently extensive to throw the balance of evidence decisively in favor of some theory of continuity, as opposed to any theory of intermittent and occasional action, which some writers have strenuously and intelligently advocated. No marks of mighty and general convulsions of nature exist, as the seeming breaks which divide the grand series of stratified rocks into numerous isolated formations would indicate. They are simply indications of the imperfection of our knowledge. Science will never, in all probability, point to a complete series of deposits, or to a complete succession of life, which shall link one geological period to another. But that such deposits and such an unbroken succession must have existed at one time we may well feel sure, and stand ready to believe that nowhere in the long series of fossiliferous rocks has there been a total break, but that there has inevitably been a complete continuity of life, as well as a more or less complete continuity of sedimentation from the Laurentian period to the present day. One generation, speaking figuratively, hands on the lamp of life to the next, and each system of rocks is the direct offspring of its predecessor in time. Though it is apparent that there has not been continuity in any given area, still the geological chain could not have been snapped at one point and taken up again at a totally different one. Hence we arrive at the conviction that in geology, as in other sciences, continuity is the fundamental law, and that the lines of demarcation between the great formations are but gaps in our own knowledge.
Through the study of fossils, as is well known, geologists have been led to the all-important generalization that the vast series of fossiliferous or sedimentary rocks may be separated into a number of definite groups or formations, each of which being characterized by its own organic remains, but not properly and strictly, it must be understood, by the occurrence therein of any one particular fossil. However, a formation may contain some particular fossil or fossils not occurring outside of that formation, thus enabling an observer to identify a given group with tolerable certainty; or, as very often happens, some particular stratum or subgroup of a series, may contain peculiar fossils, whereby its existence may be determined with considerable readiness in divers localities. Each great formation, let it be said, is properly characterized by the association of certain fossils, the predominance of certain families or orders, or by an assemblage of fossil remains that represent the life of the period during which the formation was deposited.
Fossils, then, not only enable us to determine the age of the deposits in which they are found, but they also further enable us to arrive at some very important conclusions respecting the manner in which the fossiliferous bed was deposited, and, consequently, to the condition of the particular region occupied by the bed at the period of its formation. Beds that contain the remains of animals, such as now inhabit rivers, we know to be fluviatile in their origin, and that at one time they must have either constituted actual river-beds, or been deposited by the overflowing of ancient streams. But if the beds contain the remains of mollusks, minute crustaceans or fish, such as are found to-day in lakes, then we conclude that they are lacustrine, and were deposited beneath the waters of former lakes. And, lastly, if the remains of animals such as now people the oceans are to be met with in the beds, then we know that they are marine in origin, and that they are fragments of an old sea-bottom. On the whole, the conditions under which a bed was deposited, whether in a shallow sea, in the immediate vicinity of a coast-line, or in deep water, can often be determined with considerable accuracy from the nature of the relics of the organisms which they contain. But we have thus far been dealing with the remains of aquatic animals. When, however, we consider the remains of aerial and terrestrial animals, or of plants, the determination of the conditions of deposition is not made out with such an absolute certainty. Remains of land-animals would, of course, occur in sub-aerial deposits, that is, in beds, like blown sand, accumulated upon the land, but the most of such remains of such animals are found in deposits which have been laid down in water, and hence their present position is due to the fact that their former owners were either drowned in rivers or lakes, or borne out to sea by water-channels. Animals possessed of the power of flight might also similarly find their way into aqueous deposits, but, when it is remembered that many birds and mammals habitually spent a great part of their time in the water, it is not to be wondered at that they should present themselves as fossils in sedimentary rocks. Even plants, such as have undoubtedly grown upon land, do not prove that the bed in which they are found was formed on land, for many of their remains are extraneous to the bed in which they now occur, having reached their present site by falling into lakes or rivers, or by being carried out to sea by floods or gales of winds. Still, there are many cases which obviously show that plants have grown on the very spot where we now find them. The great coal-fields of the Carboniferous Age, it is now generally conceded, are the result of the growth in situ of the plants which compose coal, as well as that they grew on vast marshy or partially submerged tracts of level alluvial land.
While fossils enable us in many cases to arrive at important conclusions as to the climate of the period in which they lived, yet it is only in the case of marine fossils, which constitute the majority of such remains, that we acquire such knowledge, but it is mostly the temperature of the sea which can thus be determined. However, let it be remembered that, owing to the existence of heated currents, the marine climate of a designated area does not necessarily imply a correspondingly warm climate in the adjoining land, for land-climates can only be determined by the relics of land-animals or land-plants, and these are comparatively rare as fossils. But all conclusions on this head are really based upon the existing distribution of vegetable and animal life upon the globe, and are therefore liable to be vitiated by the considerations that no certainty exists that the habits and requirements of an extinct animal were exactly similar to those of its nearest living relative; that far back in time groups of organisms, so unlike anything we know at the present day, are met with, which render all conjectures of climate based upon their supposed habits more or less uncertain and unsafe; that in the case of marine animals we are as yet very far from knowing the precise limits of distribution of many species within our present seas as to render conclusions drawn from living forms in relation to extinct species unsatisfactory and, probably, incorrect; and, finally, that the distribution of animals to-day, is certainly dependent on other conditions than climate alone, the causes limiting the range of given animals being assuredly such as belong to the existing order of things, and are different from what they were in former times, not necessarily because the climate has changed, but because of the alteration of other conditions that are essential to the life of the species or conducive to its extension. But notwithstanding the difficulties in the way, we are able in many cases to deduce completely trustworthy conclusions concerning the climate of a given geological period by an examination of its fossil remains. In Eocene times, or at the beginning of the Tertiary Period, the climate of what is now Western Europe was of a tropical or sub-tropical character, the Eocene beds being found to contain the remains of cowries and volutes, such shells as now inhabit tropical seas, together with the fruits of palms and remains of other tropical plants. And further, it has been shown that in Miocene times, or about the middle of the same epoch, the central parts of Europe were peopled with a luxuriant flora resembling that of the warmer parts of the United States, and that Greenland, now buried for the most part beneath a vast ice-shroud, was warm enough to support a large number of trees, shrubs and other plants that are at present denizens of the temperate regions of the globe.
And lastly, from the study of fossils, geologists first learned to comprehend a fact, that is, that the crust of the earth is liable to local elevations and subsidences, which may be regarded as of cardinal importance in all modern geological theories and speculations. Long after the remains of shells and those of other marine animals were first observed in the solid rocks constituting the dry land, and at great elevations above the sea-level, attempts were made to explain this unintelligible phenomenon upon the hypothesis that these remains or fossils were mere lusus naturæ, due to some “plastic virtue latent in the earth.” But the common-sense of science soon rejected this idea, and it was universally agreed that these bodies were really the relics of animals that once lived in the sea. When once this was admitted, further steps in the right way of thinking became comparatively easy, and at the present day no geological doctrine stands on a surer foundation than that which teaches that our existing continents and islands, fixed and immovable as they appear, have been repeatedly sunk beneath the ocean and just as repeatedly been lifted above its waters.
Not only have fossils an important bearing upon geology and physiography as has been seen, but they have relations, most complicated and weighty in character, with the science of biology, or the study of living beings. No adequate understanding of zoölogy and botany is possible without some acquaintance with the types of plants and animals that have passed away, for there are numerous speculative problems in the domain of vital science, which, if soluble at all, can only hope to find their key in researches carried out on extinct organisms.
No attempt will be made by the writer to discuss fully the biological relations of fossils. Such an undertaking would afford matter for a separate volume. All that I purpose in this chapter is to indicate very cursorily the principal points of palæontological teaching, so that my readers can acquire some idea of the progression from lower to higher types that life has made throughout the geological ages. Preliminary to the purpose held in view, let it be understood that the vast majority of fossil animals and plants are extinct, or, differently and perhaps more intelligently expressed, belong to species that no longer exist. So far from there being any truth in the old idea that there have been periodic destructions of all the living beings in existence upon the earth, followed by a corresponding number of new creations of plants and animals, the actual facts indicate that the extinction of old and introduction of new forms have been processes that have been continually going on throughout the whole of geologic time. Every species seems to come into existence at a definite point of time, and to disappear finally at another definite period, though there are few, if any, instances, in which the times of entrance and exit could be fixed with any degree of certainty or precision. Marked differences in the actual time during which different species have remained in existence are noticeable, and therefore corresponding differences in their vertical range, or in the actual amount and thickness of strata through which they present themselves as fossils, some species being found to extend through two or three formations, and even a few have had a more prolonged existence. More commonly, however, the species which begin in the commencement of a great formation die out at or before its close, while those which are introduced for the first time near its middle or end may either become extinct or pass into the next succeeding formation, animals of the lowest and simplest organization as a rule having the longest range in time. Microscopic or minute dimensions seem to favor longevity, for some of the Foraminifera appear to have survived, with little or no perceptible alteration, from the Silurian Period to the present day, whereas largely and highly-organized animals, though long-lived as individuals, rarely seem to live long specifically, and consequently have a restricted vertical range. Exceptions to this rule are, however, occasionally found in some persistent types, the Lampshells of the genus Lingula being little changed from the Lingulæ that swarmed in the Lower Silurian seas, while the existing Pearly Nautilus is the last descendant of a clan nearly as old. Some forms, on the other hand, the Ammonites, which are closely related to the Nautilus, and mostly restricted to certain zones of strata, seem to have enjoyed a comparatively brief lease of life.
LIFE IN THE PRIMORDIAL SEA.
Representing Mollusks, Sponges, Crustaceans, Worms and Sea-Weeds.
But of the causes that have led to the extinction of plants and animals, little or nothing is known. All that can be affirmed, in our present knowledge, is that the attributes constituting a species do not seem to be intrinsically endowed with permanence, any more than those constituting an individual, though the former may endure whilst many successive generations of the latter have disappeared from the earth. Each species, it would seem, has its own life-period—its beginning, culmination and decay—the life-periods of different species being of very different duration. From all that has been said, it may be gathered that our existing plants and animals are for the most part of modern origin, using the term modern in its geological acceptation. Measured by human standards, many of our existing animals, those which are capable of being preserved as fossils, are known to have a high antiquity. Not a few of our shell-fish commenced their existence at some time in the Tertiary, while one species of Lampshell—Terebratulina caput-serpentis—is believed to have survived since the Chalk, and a number of the Foraminifera date from the Carboniferous Period. Thus, we learn the additional fact that our existing flora and fauna do not constitute an aggregation of organic forms which were introduced into the world collectively and simultaneously, but that they commenced their existence at very different times, some being extremely ancient, whilst others are of comparatively recent origin. And this introduction of existing plants and animals, as admirably shown by the study of the fossil shells of the Tertiary Period, was a slow and gradual process. Ninety-five per cent. of the known fossil shells in the earliest Tertiary are found to be species no longer in existence, the remaining 5 per cent. being forms that are known to live in our present seas. In the Middle Tertiary, the extinct types are much fewer in number, while at the close of the Period the proportion with which we started may be reversed, not more than 5 per cent. being extinct types.