In reviewing the facts concerning humanity, which are well authenticated at the present date, with the object of getting a composite view of the greatest of all “world riddles”—“Life”—possibly nothing tends so largely to expand our mental horizon as a study of the earth itself or man’s place of abode. The ideas of the educated and cultured mind, at the beginning of the twentieth century, upon cosmogony, are necessarily of such a character that man’s heretofore undisputed boast of being the objective and acme of creation or evolution is forced into that great mass of theories which science has proven to be absolutely untenable. Since the relative importance of the factors of heredity and adaptation has become known, the environment, or conditions surrounding man’s existence in times past, is of exceptional importance, as, from an understanding of these prehistoric limitations, we are better able to judge what must have been the achievement of the individual and the race than we could be when in ignorance of these facts.

The length of prehistoric time (so far as our earth is concerned) has been the subject of much intelligent labor and thought, as well as the occasion for much dissenting of opinion and more or less designed misstatement. Until very recently, it has been difficult to reconcile the theories, as promulgated by the authorities in the various departments of science; but, notwithstanding this, some light may be obtained by the summarization of the most plausible hypotheses now advocated. We cannot take the space to go into detail concerning these, but will merely touch upon the most salient points.

The constancy of the supply of heat furnished by the sun and the division of the year into definite seasons was one of the first phenomena which attracted the attention of man at the dawn of history, and in the many accounts of the creation which we find in literature we see the feeble attempts of man to account for what he observed. Although the knowledge which we have at the present time is not complete enough to warrant any feeling of pride, yet we do know enough to say, with certainty, some things concerning the solar system. We know that our sun cannot forever radiate away its heat into space without sometime becoming as cold or colder than we are, unless the energy which it is losing in the form of heat be restored to it by some means not at this time known. Sir William Thomson (Lord Kelvin) has calculated that at the present rate of solar radiation, which amounts to about twenty-eight calories per minute, per square centimeter, at the distance of the mean radius of the earth’s orbit, it would have taken somewhat more than fifteen million years for the heat generated by the contraction of the sun’s mass from the orbit of the outer planet, Neptune, to its present size, to have been radiated away into space. This means that gravity, as a source of heat development, at the rate of solar radiation now known, would account for, perhaps, twenty million years’ expenditure of energy in reducing the sun’s diameter to but one-thirteen-thousandth part of what it once was. Not only does the nebular hypothesis fall short of accounting for the facts, as will subsequently be shown in this one particular of the length of time during which our solar system has existed, but it does not account for the variation in the obliquity of the poles of the planets, which are the attendants upon the sun; nor does gravitative attraction alone enable us to account for the tremendous velocities of some of the stars through space, such as Arcturus,—so that it may be safely assumed that we shall be forced to modify our ideas as to the value of the nebular hypothesis as a working basis, before we can harmonize our deductions from astronomical and geological grounds. Fortunately, the study of the spiral nebulÆ has done much to elucidate our conceptions of the formation of the planetary systems, and from the discoveries made concerning these highly attenuated bodies of matter, a new hypothesis has been formed which will completely harmonize, perhaps, with these above stated facts, which could not be made to accord with the nebular theory as previously held.

One source of the continued acquisition of energy by our sun, whose value is hard to estimate, is the shooting stars, or meteors, which constantly fall into it. Astronomical records show that, from the earth alone, no less than twenty million shooting stars are daily within the limits of vision, and inasmuch as the solar system is moving with a velocity of some twenty miles per second through space, it will be seen that the number of meteors which would come within the influence of the sun, being as it is about one and one-third million times the volume of the earth, would be practically infinite. What then must be said of the amount of energy acquired by the sun from these, although each meteor may have a mass of but a few grams, and perhaps may be only several hundred miles away from its successor? It is clearly demonstrated that, if no such additions of energy were received by our sun, in about ten million years its diameter would be reduced to one-half of what it is now, and its mass, where now it exists as a gas, would then become a solid, at least upon the surface, and the quantity of heat received by the earth would become so small that life here, as we know of it, would be an impossibility. But if it be granted that the sun annually gathers, by its gravitative attraction, a combined mass of matter equal to the one-hundredth part of our earth, at a distance away from its center equal to the main radius of the earth’s orbit, the energy dissipated by its radiation of heat at its present rate would be accounted for, while the sensible heat of the sun would not diminish, and the supply would be kept up indefinitely. That such additions of mass are made, there can be no doubt, but as to their quantity, we cannot, with our present knowledge, even hazard a guess.

In speaking of the solar heat and man’s dependence upon it in a constant definite quantity, as one of the conditions of his existence, perhaps it will give us some just appreciation of his place in nature when we consider that the earth receives somewhat less than one two-billionth part of the heat radiated away by the sun, and while this expression makes the quantity which we receive seem rather small, it is, nevertheless, large enough annually to melt a layer of ice one hundred and seventy-five feet thick—all over the surface of the earth, and is a little more than one six-thousandth part of the quantity of heat which would be generated by the burning of a mass of coal as large as the sun.

The researches of Halley and Adams have shown that from some cause, probably the result of gravity acting in conjunction with the varying eccentricity of the earth’s orbit, the motion of the moon has been slightly accelerated as time went on, while the diurnal motion of the earth has been reduced by the action of the tides, and that the amount of this loss, in time, is equal to about one second in the length of our day, in 168,000 years. Now, this retardation in the earth’s motion has not taken place at a uniform rate if caused by the reaction of the tides, as the nearer to the earth the moon was, the greater would be the tides, and, consequently, the greater would be the reaction; i.e., the retardation. But assuming that this retardation took place, on the whole, at twice the rate now prevailing, we would still have a period of six million years since the moon was thrown off by the earth, when our days were but three hours long.

Turning from the theories of astronomy, which are obviously more or less inaccurate, owing to their very nature and the character and duration of the observations upon which they are based, we come to the nearer and more certain deductions of geology. Here we have the phenomena of denudation and deposition with which to deal, and inasmuch as these are measurable at many places, and under many conditions upon the earth to-day, it is safe to assume that computations made from these measurements cannot be far from the truth. We know that practically all of the great formations of the earth were depositions of material from water which contained them, and that, in many cases, heat caused these strata to be metamorphosed or crystallized ages after they were deposited, and that in this crystallization many of the fossils remaining imbedded in the deposited matter were destroyed. Concerning this deposition we know that it is going on to-day in the Atlantic and Pacific Oceans, where, in the deeper portions the Globigerina ooze is filling in these depressions with a deposit, resembling chalk, at the rate of perhaps an inch per century. We know that the Gulf of Mexico and several other ocean areas are being filled in with silt at the rate of as high as three inches per century. This silt is brought down in the tributary rivers and emptied into the gulfs. We also know that large areas in the Indian Ocean are being covered with coral and the dÉbris from the coral reefs. We are absolutely certain that every geological period has had its characteristic fauna and flora, and that, in both the animal and vegetable kingdoms, some persistent types have connected it with both the past and the future, so that the fossils have become the “open sesame” to the geological records. We further know that the strata composing the earth’s surface are subject to elevation and subsidence, such as is now going on in the delta of the Nile, on the coast of the Netherlands, and in many other places, and that such movement is a measurable quantity, given only the necessary time.

The total thickness of known strata measures but about one-three hundred and twentieth part of the earth’s diameter, or, in round numbers, twenty-five miles. Thirty thousand feet of this is quite readily identified as belonging to the old Archaic or Laurentian period, and constitutes the oldest stratified deposit known. Even in this, we find the remains of the Eozoon Canadense, which is now universally acknowledged to be the petrifaction of a foraminiferous living organism with a chambered shell. This means that, at this time, the earth’s atmosphere must have been very similar to what it is at the present, and that the temperature of the sea was somewhere between the boiling and the freezing points of water. What time had elapsed since the earth was thrown off by the sun in an incandescent state can only be faintly imagined. At the rate of deposition given for the deepest of ocean deposits, this Archaic period would have taken perhaps thirty-six million years; but inasmuch as the water may have been far warmer then than now, and the rainfall more abundant, and the forces of denudation in all respects more active, this figure may be excessive. The next eighteen thousand feet of strata are easily identified as Lower Silurian, by the Diatoms which occur imbedded in them, and these formations include some of the largest deposits of limestone known. At our rate of calculation, this deposit would require no less than nine and one-half million years, and, in assuming this figure, no account is made of the intervals of time during which no deposit took place, although such periods of inactivity must necessarily have been. The Upper Silurian strata consists of twenty thousand feet, the fossils of which are the lower fishes, and for which we must assign a period of time equal to no less than twenty-five million years, inasmuch as these deposits are limestones and sandstones, or the remains of water-living animals and plants.

Coming now to the Devonian and Carboniferous periods, the strata of the former, which is filled with fossils of the dipnoi, and the latter with those of the amphibia; we have deposits aggregating about forty thousand feet, and inasmuch as long intervals of time must have existed during the subsidence and elevation, and vice versa, of the land, while the process of coal-forming was going on, it is certain that our rate of deposition as heretofore used, is entirely too high. Dawson and Huxley have estimated, after most careful investigation, that the period of time consumed in laying down the coal measures, could not be less than six million years, and upon this basis it is safe to assume that between seventy-five and eighty million years were consumed in laying down the Devonian and Carboniferous deposits. This makes Paleozoic time occupy about one hundred and fifty million years, which is probably under- rather than over-estimated. The flora of the Carboniferous period was composed of tree ferns of the Sagillaria and Lepidodendron species which have since become extinct; but the Lingula, a shell in the Cambrian and Upper Silurian formations, and the Terbratula, another shell, is found in the Devonian rocks. Both of these are found living to-day, of the same identical genus and species.

In the Silurian rocks, we find the remains of an air-breathing scorpion, very similar to that found to-day, which shows that the atmosphere at that remote period was practically the same as we have at the present time.

In the Mesozoic time, we find deposits aggregating some fifteen thousand feet, and inasmuch as the Triassic sandstones were formations of slow deposition, our heretofore established rate will not answer the conditions. It has been estimated, after the most careful study of the Triassic and Jurassic measures, that probably no less than thirty million years were occupied by these periods, and that the chalk deposits of the Cretaceous must have taken at the present known rate, in like formations, somewhat over six million years of ceaseless activity. This gives to Mesozoic time a period of thirty-six million years, as a minimum, and, from what we know of the rate of biological evolution, this figure is conservative. The first period of the Mesozoic time was characterized by monotremes, the Jurassic by marsupials, and the latter by the first of man’s direct progenitors, the placentals. The flora of this period consisted almost entirely of gymnosperms, or naked seed plants, and, as far as we know, at the close of this second great division of geological time, conditions on the earth were, in all respects, very much as they are to-day.

Concerning the climatic conditions at the beginning of the Cenozoic time, we have every reason to believe that from the commencement of the Lower Silurian epoch, until then, there were no climatic zones upon the earth. Not only have coral formations been found in what are now Arctic waters, when we know that such reefs are formed only in waters where a moderately warm temperature is constantly maintained, but the cephalipods of the genus Ammonitoidea are found in what is now the Antarctic zone, and in the torrid. While, at the present time, we cannot see how the obliquity of the earth’s poles to the plane of the ecliptic could have been changed after the earth began its career as an independent planet, yet the facts above stated show that the climatic zones must have been unknown during the Tertiary period. Our common cypress, which is now so plentiful in Florida and California, had very close relatives living as far north as Spitzbergen, as lately as Miocene time. Magnolias, which are now so abundant in all of the Gulf States, are plentifully found in the Miocene strata of Greenland.

Returning to the length of the Tertiary period, it is well to note that, covering Wyoming and Nebraska, there was an immense lake, at least as large as Lake Superior is to-day, and into which several quite large rivers emptied, whose head waters were in the surrounding mountain ranges. This lake was at one time at least five thousand feet deep, and was completely filled up by the fine mud and silt, as the formation now shows, although at the known rate of filling in of smaller modern lakes, into which rivers, which originate in glaciers, empty, this would have taken the better part of fifty thousand years. This figure is particularly conservative, as during the Eocene period, there could have been neither glaciers nor melting snowfields to assist in the denudation at the head waters of the tributary rivers. During the Miocene period, many of the best geologists hold that America and Europe were connected, and there are certain similarities in their fauna and flora which make this very probable. Supposing that this depression which constitutes the bed of the North Atlantic Ocean, took place at the highest known rate of subsidence, as measured upon the coast of Sweden to-day, it is almost impossible to state the amount of time that necessarily elapsed from the beginning of the sinking of this strip until it finally went below the surface of the water. That such changes in level did take place in the Tertiary period, no one can doubt, as chalk deposits in England, which must have been laid down in the deep oceans, have now an elevation of thousands of feet. The Nummulite limestone of this same period is found in both the Alps and the Himalayas, at an elevation as great as ten thousand feet. The consideration of the fact that the greatest known rate of elevation or subsidence is, perhaps, scarcely more than two feet per century makes the figure of five hundred thousand years, as a minimum for Pliocene time, seem rather conservative.

Toward the close of the Tertiary era the finishing touches were placed upon some of the greatest of the geological works. The folding of the strata, which had been going on for a long period in Eastern New York, was brought to an end by a violent rupture therein, and the out-rushing igneous rock, which was subsequently cooled rapidly by the floods of water flowing over it, gave us the beautiful palisades of the Hudson River. In the west, this folding resulted in the Rocky Mountains and the Coast Range, with their attendant high plateaux. In Europe, the Alps and the Pyrenees Mountains both belong to this period, while the grandest and highest of all mountain chains, the Himalayas, of Asia, were the culminating effect of the gigantic foldings of the earth’s crust.

The deposits of the Tertiary period will aggregate somewhat more than three thousand feet, and, inasmuch as this entire time was one of continued change in level, or the fluctuation between the subsidence of the earth’s strata on the one hand and the elevation on the other (particularly in the Pliocene period), it is very hard to form any conjecture as to the actual amount of time required to do this work. Certainly, from what we know of the rate at which like phenomena are taking place at the present time in Northeastern North America, in Northwestern Europe, and Western Asia, the figure, as sometimes given, of ten million years seems very conservative.

In the brief review which we have just given, of what can be conservatively considered the minimum limits of geological time, we have taken into account generally only periods of activity, and in but a few cases has any estimation been hazarded as to the proportion which this was of the whole time consumed in bringing about the changes which the fossils show so clearly to have taken place during the various epochs. But one thing should be kept clearly in mind, and that is, that no matter how long geological time may seem, it is but an infinitely small fraction of the period which must have elapsed since the world came into existence, as this globe had to cool down to below the boiling point of water before any geological records could be made. When thought of in this way, the Laurentian period becomes as but yesterday, and even man’s dwelling place, which seems relatively so large, dwindles into nothingness, when compared with the vastness of the interstellar spaces or the size of the larger stars. Whoever conscientiously endeavors to form any idea of the teachings of astronomy and geology, must necessarily feel any prejudice which he had for man as the object and culmination of either the evolutionary or creative power, shrink at a tremendous rate, while over his mentality comes the sense of his diminutiveness, which awakens in him a brotherly feeling for even the primitive single-celled Laurentian Eozoon Canadensis, or the unnucleated monera of the present time. It must have been this same sense-perception in the Hindoos which made them worship and revere life wherever they found it, and which inspired them with so active a sympathy toward all living things.