Chapter 7

What, now, are the reasons why the palæontological evidence is not complete and why it cannot be? In the first place the seeker after fossil remains finds about three fifths of the earth's surface under water so that he cannot explore vast areas of the present ocean beds which were formerly dry land and the homes of now extinct animals. Thus the field of investigation is seriously restricted at the outset, but the naturalist finds his work still more limited, in so far as much of the dry land itself is not accessible. The perennial snows of the Arctic region render it impossible to make a thorough search in the frigid zone, and there are many portions of the temperate and torrid zones that are equally unapproachable for other reasons. But even where exploration is possible, the surface rocks are the only ones from which remains can be readily obtained, for the layers formed in earlier ages are buried so deeply that their contents must remain forever unknown in their entirety. Only a few scratches upon the earth's hard crust have been made here and there, so it is small wonder that the complete series of extinct organisms has not been produced by the palæontologist.

A brief survey of the varied groups of animals themselves is sufficient to bring to light many biological reasons which account for still more of the vacant spaces in the palæontological record. We would hardly expect to find remains of ancient microscopic animals like the protozoa, unless they possessed shells or other skeletal structures which in their aggregate might form masses like the chalk beds of Europe. Jellyfish and worms and naked mollusks are examples of the numerous orders of lower animals having no hard parts to be preserved, and so all or nearly all of the extinct species belonging to these groups can never be known. But when an animal like a clam dies its shell can resist the disintegrating effects of bacteria and other organic and inorganic agencies which destroy the soft parts, and when a form like a lobster or a crab, possessing a body protected by closely joined shell segments, falls to the bottom of the sea, the chances are that much of the animal's skeleton will be preserved. Thus it is that corals, crustacea, insects, mollusks, and a few other kinds of lower forms constitute the greater mass of invertebrate palæontological materials because of their supporting structures of one kind or another. Perhaps the skeletal remains of the vertebrates of the past provide the student of fossils with his best facts, on account of the resistant nature of the bones themselves, and because the backboned animals are relatively modern; then, too, the rocks in which their remains occur have not been so much altered by geological agencies, or buried so deeply under the strata formed later. Of course only the hardest kinds of shells would remain as such after their burial in materials destined to turn into rock; in the majority of cases, an entombed bone is infiltrated or replaced by various mineral substances so that in time little or nothing of the original thing would remain, though a mold or a cast would persist.

But even if an animal of the past possessed hard structures, it must have satisfied certain limited conditions to have its remains prove serviceable to students of to-day. A dead mammal must fall upon ground that has just the right consistency to receive it; if the soil is too soft, its several parts will be separated and scattered as readily as though it had fallen upon hard ground where it would be torn to pieces by carnivorous animals. The dead body must then be covered up by a blanket of silt or sand like that which would be deposited as the result of a freshet. If a skeleton is too greatly broken up or scattered, it may be difficult or even impossible for its discoverer to piece together the various fragments and assemble them in their original relations. Very few individuals have been so buried and preserved as to meet the conditions for the formation of an ideal fossil. To realize how little may be left of even the most abundant of higher organisms, we have only to recall that less than a century ago immense herds of bison and wild horses roamed the Western plains, but very few of their skulls or other bones remain to be enclosed and fossilized in future strata of rocks. When we appreciate all these difficulties, both geological and biological, we begin to see clearly why the ancient lines of descent cannot be known as we know the path and mode of embryonic transformation. The wonder is not that the palæontological record is incomplete, but that there is any coherent and decipherable record at all. Yet in view of the many and varied obstacles that must be surmounted by the investigator, and the adverse factors which reduce the available evidence, the rapidly growing body of palæontological facts is amply sufficient for the naturalist to use in formulating definite and conclusive principles of evolution.

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For the purposes of palæontology, the most essential data of geology are those which indicate the relative ages of the strata that make up the hard outer crust of the earth, for only through them can the order of animal succession be ascertained. It does not matter exactly how old the earth may be. While it is possible to determine the approximate length of time required for the construction of sedimentary rocks like those which natural agencies are producing to-day, there are few definite facts to guide speculation as to the mode or duration of the process by which the first hard crystalline surface of the earth was formed. But palæontology does not care so much about the earliest geological happenings, for it is concerned with the manifold animal forms that arose and evolved after life appeared on the globe. Questions as to the way life arose, and as to the earliest transformations of the materials by which the earth was first formed are not within the scope of organic evolution, although they relate to intensely interesting problems for the student of the process of cosmic evolution.

According to the account now generally accepted, the original material of the earth seems to have been a semi-solid or semi-fluid mass formed by the condensation of the still more fluid or even gaseous nebula out of which all the planets of the solar system have been formed and of which the sun is the still fiery core. As soon as the earth had cooled sufficiently its substances crystallized and wrinkled to form the first mountains and ridges; between and among these were the basins which soon filled with the condensing waters to become the earliest lakes and oceans. The wear and tear of rains and snows and winds so worked upon the surfaces of the higher regions that sediments of a finer or coarser character like sand and mud and gravel were washed down into the lower levels. These sediments were afterwards converted into the first rocks of the so-called stratified or sedimentary series, as contrasted with the crystalline or plutonic rocks like the original mass of the earth and the kinds forced to the surface by volcanic eruptions. Later the earth wrinkled again in various ways and places so that new ridges and mountains were formed with new systems of lakes and oceans and rivers; and again the elements continued to erode and partially destroy the higher masses and to lay down new and later series of sedimentary rocks upon the old.

It seems scarcely credible that the apparently weak forces of nature like those we have mentioned are sufficiently powerful to work over the massive crust of the earth as geology says they have. Our attention is caught, as a rule, only by the greater things, like the earthquakes at San Francisco and Valparaiso, and the tidal waves and cyclones of the South Seas; but the results of these sporadic and local cataclysms are far less than the effects of the persistent everyday forces of erosion, each one of which seems so small and futile. When we look at the Rocky Mountains with their high and rugged peaks, it seems almost impossible that rain and frost and snow could ever break them up and wear them down so that they would become like the rounded hills of the Appalachian Mountain chain, yet this is what will happen unless nature's ways suddenly change to something which they are not now. A visitor to the Grand Cañon of the Colorado sees a magnificent chasm over a mile in depth and two hundred miles long which has actually been carved through layer after layer of solid rock by the rushing torrents of the river. Perhaps it is easier to estimate the geological effects of a river in such a case as Niagara. Here we find a deep gorge below the famous falls, which runs for twenty miles or so to open out into Lake Ontario. The water passing over the brim of the falls wears away the edge at a rate which varies somewhat according to the harder or softer consistency of the rocks, but which, since 1843, has averaged about 104 inches a year. Knowing this rate, the length of the gorge, and the character of the rocky walls already carved out, the length of time necessary for its production can be safely estimated. It is about 30,000 to 40,000 years, not a long period when the whole history of the earth is taken into account. A similar length of time is indicated for the recession of the Falls of St. Anthony, of the Mississippi River, an agreement that is of much interest, for it proves that the two rivers began to make their respective cuttings when the great ice-sheet receded to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these gorges? As gravel and mud and silt the detritus has been carried to the still waters of the lower levels, to be laid down and later solidified into sandstone and slate and shale. All over the continents these things are going on, and indefatigable forces are at work that slowly but surely shear from the surface almost immeasurable quantities of earth and rock to be transported far away. In some instances it is possible to find out just how much effect is produced in a given period of time, especially in the case of the great river systems. For example, the mass of the fine particles of mud and silt carried in a given quantity of the water of the Mississippi as it passes New Orleans can be accurately measured, and a satisfactory determination can also be made of the total amount of water carried by in a year. From these figures the amount of materials in suspension discharged into the Gulf of Mexico becomes known. It is sufficient to cover one square mile to the depth of 269 feet; in twenty years it is one cubic mile, or five cubic miles in a century. Turning now to the other aspect of this process, and the antecedent causes which produce these effects, it appears that the area of the Mississippi River basin is 1,147,000 square miles--about one third of the total area of the United States. Knowing this, and the annual waste from its surface, it is easy to demonstrate that it will take 6000 years to plane off an average of one foot of soil and rock from the whole of this immense area. Of course only an inch or a few inches will be taken from some regions where the ground is harder or rockier, or where little rain falls, while many feet will be washed away from other places. The waters of the Hoang-ho come from about 700,000 square miles of country, from which one foot of soil is washed away in 1464 years. The Ganges River, draining about 143,000 square miles, carries off a similar depth of eroded materials from its basin in 823 years! Should we add to the above figures those that specify the bulk of the chemical substances in solution carried by these waters, the total would be even greater. We know that in the case of the Thames River, calcareous substances to the amount of 10,000 tons a year are carried past London, and all this mineral has been dissolved by rain-water from the chalky cliffs and uplands of England, so that the land has become less by this amount. Thus we learn that vast alterations are being made in the structure of great continents by rain and rivers, as well as by glaciers and other geological agencies. And at the same time that old strata are undergoing destruction new ones are in process of construction at other places, where animal remains can be embedded and preserved as fossils. The forces at work seem weak, but they continue their operations through ages that are beyond our comprehension and they accomplish results of world-building magnitude.

Thus the whole process of geological construction is such that older exposed strata continually undergo disintegration, but this involves the destruction of any fossils that they might contain. The very forces that preserve the relics of extinct animals at one time undo their work at a later period. There are many other influences besides that destroy the regularity of rock layers or change their mineralogical characters by metamorphosis. It is easier to see how volcanic outbursts alter their neighboring territory. The intense subterranean heat and imprisoned steam melt the deeper substances of the earth's crust, so that these materials boil out, as it were, where the pressure is greatest, and where lines of fracture and lesser resistance can be found. Because so much detritus is annually added to the ocean floors--enough to raise the levels of the oceans by inches in a century--it is natural that greater pressures should be exerted in these areas than in the slowly thinning continental regions. These are some of the reasons why volcanoes arise almost invariably along the shores or from the floors of great ocean beds. The chain that extends from Alaska to Chili within the eastern shore of the Pacific Ocean, and the many hundreds of volcanoes of the Pacific Islands bring to the surface vast quantities of eruptive rocks which break up and overlie the sedimentary strata formed regularly in other ways and at other times. The volcanoes of the Java region alone have thrown out at least 100 cubic miles of lava, cinders, and ashes during the last 100 years--twenty times the bulk of the materials discharged into the Gulf of Mexico by the Mississippi River in the same period of time.

From these and similar facts, the naturalist finds how agencies of the present construct new rocks and alter the old; and so in the light of this knowledge, he proceeds with his task of analyzing the remote past, confident that the same natural forces have done the work of constructing the lower geological levels because these earlier products are similar to those being formed to-day. After learning this much, he must immediately undertake to arrange the strata according to their ages. This might seem a difficult or even an impossible task, but the rocks themselves provide him with sure guidance.

Wherever a river has graven its deep way through an area of hard rocks, as in the case of Niagara, the walls display on their cut surfaces a series of lines and planes showing that they are superimposed layers formed serially by deposits that have differed some or much at different times according to the circumstances controlling the erosion of their constituent particles. A layer of several feet in thickness may be composed of compact shale, while above it will be a zone of limestone, and again above this another layer of shale. Successive strata like these, where they are parallel and obviously undisturbed, are evidently arranged in the order of their formation and age. But by far the most impressive demonstration of the basic principle of geology employed for the determination of the relative ages of rocks is the mighty Cañon of the Colorado. As the traveler stands on the winding rim of this vast chasm, his eye ranges across 13 miles of space to the opposite walls, which stretch for scores of miles to the right and left; upon this serried face he will see zone after zone of yellow and red and gray rock arranged with mathematical precision and level in the same order as on the steep slopes beneath him. Plain common sense tells him that the great sheets of rock stretched continuously at one time between the now separate walls, and that the various strata of sandstone and limestone were deposited in successive ages from below upwards in the order of their exposure. When now he extends his explorations to another state like Utah or Wyoming, he may find some but not all of the series exhibited in the Grand Cañon, overlaid or underlaid by other strata which in their turn can be assigned to definite places in the sequence. By the same method, the geologist correlates and arranges the rocks not only of different parts of the same state, or of neighboring states, but even those of widely separated parts of North America and of different continents. But he learns that he must refrain from over-hasty conclusions, for he soon finds that the sedimentary rocks have not been constructed at the same rate in different places during one and the same epoch, and that rocks formed even at one period are not always identical in nature. But his guiding principle is sensible and reasonable, and by employing it with due caution he provides the palæontologist with the requisite knowledge for his special task, which is to arrange the extinct animals whose remains are found as fossils of various earth ages in the order of their succession in time.

CONDENSED TABLE OF PALAEONTOLOGICAL FACTS

__________________________________________________________________________ | | | | YEARS | NUMBER OF | | | ORDER OF NECESSARY FOR | FEET IN | GEOLOGICAL | GEOLOGICAL | APPEARANCE OF FORMATION | THICKNESS | AGE | EPOCH | CHARACTERISTIC | | | | GROUPS ______________|___________|______________|_______________|________________ | | | | | | | | M B R A F I | | | | a i e m i n b | | | | m r p p s v r | | Recent | | m d t h h e a | | or | | a s i i e r t | | Quaternary | | l l b s t e | | | | s e i e s | | | | s a - ______________|___________|______________|_______________|_|_|_|_|_|_|____ | | | | | | | | | | | | | Pleistocene | | | | | | | | | Cenozoic | Pliocene | | | | | | | 5,000,000 | 25,000 | or | Miocene | | | | | | | | | Tertiary | Oligocene | | | | | | | | | | Eocene | | | | | | | ______________|___________|______________|_______________|_|_|_|_|_|_|____ | | | | | | | | | | | | Mesozoic | Cretaceous | | | | | | | 4,000,000 | 23,000 | or | Jurassic | | | | | | | | | Secondary | Triassic | | | | | | ______________|___________|______________|_______________|_____|_|_|_|____ | | | | | | | | | | | Permian | | | | | | | Palæozoic | Carboniferous | | | | 21,000,000 | 106,000 | or | Devonian | | | | | Primary | Silurian | | | | | | Cambrian | | | ______________|___________|______________|_______________|________________ | | | | 20,000,000 | 30,000 | Azoic | Archæn | ______________|___________|______________|_______________|________________

After what seems an unduly long preparation, we now come to the actual biological evidence of evolution provided by the results of this division of zoölogical science. But all of the foregoing is fundamentally part of this department of knowledge and it is absolutely essential for any one who desires to understand what the fossils themselves demonstrate.

The oldest sedimentary rocks are devoid of fossil remains and so they are called the Azoic or Archæan. They comprise about 30,000 feet of strata which seem to have required at least 20,000,000 years for their formation. This period is roughly two-fifths of the whole time necessary for the formation of _all_ the sedimentary rocks, and this proportion holds true even if the entire period of years should be taken as 100,000,000 instead of 50,000,000 or less. The earth during this early age was slowly organizing in chemical and physical respects so that living matter could be and indeed was formed out of antecedent substances--but this process does not concern us here. The important fact is that the second major period, called the Palæozoic, or "age of ancient animals," saw the evolution of the lowest members of the series,--the invertebrates,--and the most primitive of the backboned animals, like fishes and amphibia. The rocks of this long age include about 106,000 feet of strata, demanding some 21,000,000 or 22,000,000 years for their deposition. Thus it is proved that the invertebrate animals were succeeded in time by the higher vertebrates, which is exactly what the evidences of the previous categories have shown. When we remember that the lower animals are devoid as a rule of skeletal structures that might be fossilized, and when we recall the fact that the strata of the palæozoic provided the materials out of which the upper layers were formed afterwards, we can understand why the ancient members of the invertebrate groups are not known as well as the later and higher forms like vertebrates. Yet all the fossils of these relatively unfamiliar creatures clearly prove that no complex animal appears upon a geological horizon until after some simple type belonging to a class from which it may have taken its origin; in brief, there are no anachronisms in the record, which always corresponds with the record written by comparative anatomy, wherever the facts enable a comparison to be made.

But the extinct animals of the third and fourth ages are more interesting to us, because there are more of them and because they are more like the well-known organisms of our present era. These two ages are called the Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so named because it was a transitional age of animals that are intermediate in a general way between the primitive forms of the preceding age and those of the next period; the latter name means the "recent-animal" age, when evolution produced not only the larger groups of our present animal series, but also many of the smaller branches of the genealogical tree like orders and families to which the species of to-day belong.