On this series of groups, and the succession of living beings, Sir. C. Lyell remarks "It is not pretended that the principal sections called Primary, Secondary, and Tertiary are of equivalent importance, or that the subordinate groups comprise monuments relating to equal portions of time or of the earth's history. But we can assert that they each relate to successive periods, during which certain animals and plants, for the most part peculiar to their respective eras, flourished, and during which different kinds of sediment were deposited."
We have already, in previous chapters, noticed the parallelism of the succession of life in the earth as revealed in Genesis with that disclosed by geology; but this subject must be farther referred to in the sequel, and in the mean time the reader may compare for himself the succession of life in the table with that in the later creative days.
5. The lapse of time embraced in the geological history of the earth is enormous. Fully to appreciate this it is necessary to study the science in detail, and to explore its phenomena as disclosed in actual nature. A few facts, however, out of hundreds which might have been selected, will suffice to indicate the state of the case. The delta and alluvial plain of the Mississippi have an area of more than 12,000 square miles, and must have an average depth of about 800 feet. At the present rate of conveyance of sediment by the river, it has been calculated that a period of about 33,000 years is implied in the deposition of this comparatively modern formation. [143] To be quite safe, let us take 30,000 years, and add 50,000 more for the remainder of the Post-pliocene or Quaternary. We may then safely multiply this number by forty, for the length of the Tertiary period. We may add three times as much for the Mesozoic period, and this will be far under the truth. It will then be quite safe to assume that the Palæozoic period was three times as long as the Mesozoic and Tertiary together. This would give altogether, say, 51,280,000 years for the whole of geological time from the beginning of the Palæozoic, leaving the duration of the Eozoic and previous periods undetermined, but requiring perhaps nearly as much time. Great though these demands may seem, they would be probably far below the rigid requirements of the case were it not for the probability that the present rate of transference of material by the great river is less than it was in Post-pliocene and early modern times. This might enable us to reduce our estimate considerably within the scope of a hundred millions of years. [144] Take another illustration from an older formation. An excellent coast section at the Joggins, in Nova Scotia, exhibits in the coal formation proper a series of beds with erect trunks and roots of trees in situ, amounting to nearly 100. About 100 forests have successively grown, partially decayed, and been entombed in muddy and sandy sediment. In the same section, including in all about 14,000 feet of beds, there are 76 seams of coal, each of which can be proved to have taken more time for its accumulation than that required for the growth of a forest. Supposing all these separate fossil soils and coals to have been formed with the greatest possible rapidity, forty thousand years would be a very moderate calculation for this portion of the Carboniferous system; and for aught that we know thousands of years may be represented by a single fossil soil. But this is the age of only one member of the Carboniferous system, itself only a member of the great Palæozoic group, and we have made no allowance for the abrasion from previous rocks and deposition of the immense mass of sandy and muddy sediment in which the coals and forests are imbedded, and which is vastly greater than the deltas of the largest modern rivers.
Considerations of a physical rather than of a geological nature also give us long periods for the probable existence of the earth, though they serve to correct somewhat the extravagant estimates of some theorists. Croll has based an interesting calculation on the amount of erosion of the land by rivers. That of the Mississippi amounts to one foot in 6000 years. That of the Ganges gives one foot in 2358 years, the average being, say, one foot in 4179 years. Some smaller rivers give a much shorter time; but the average of two great rivers, one draining a very large area of the western and another of the eastern hemisphere, and in very different climates and geographical conditions, will probably be the most reliable datum. Croll, however, prefers the Mississippi rate. [145] If we estimate the proportion of land to water as 576 to 1390, this will give for the entire area of the ocean a rate of deposition of one foot in 14,400 years. Now the entire thickness of all the stratified rocks is estimated at 72,000 feet; and at this rate the enormous time of 1,036,800,000 years would be necessary. But we have no right to assume that deposition has been going on uniformly over the entire sea-bottom. On the contrary, the greater part of it takes place within a belt of about one hundred miles from the coasts, and the deposit of calcareous and other matters over the remainder will scarcely make up for the portions of this belt on which no deposit is taking place. This will give an area of deposit of about 11,650,000 square miles, consequently only one twelfth of the above time, or about 86,400,000 years, would be required. This can be but a very rough calculation; but it has the merit of squaring very nearly with the calculations derived from physical considerations, more especially by Sir William Thomson, which limit the possible existence of the earth's solid crust to one hundred millions of years. Similar conclusions have also been deduced from what is known of the physical constitution of the sun. Croll's own ingenious theory of glacial periods produced by the varying eccentricity of the earth's orbit, along with the precession of the equinoxes, would give, according to him, about 80,000 years ago for the date of the Glacial period, and for the beginning of the Tertiary period about 3,000,000 years ago.
It would thus appear that physical and geological science conspire in assigning a great antiquity to the earth, but not an unlimited antiquity. They agree in restricting the ages that have elapsed since the introduction of life within one hundred millions of years. I confess, however, that a consideration of the fact that all our geological measures of erosion and deposition seem to be based on cases which refer to what may be termed minimum action leads me to believe that the actual time will fall very far within this limit. For example, if we were to suppose an elevation of the land drained by the Mississippi even to a small amount, its cutting power would be vastly increased for a long time. The same effect would result from a subsidence and re-elevation, or from any cause increasing the amount of rainfall or deposition of snows in winter. Now we know that such things have occurred in the past, while we have no reason to believe that the amount of action was ever much less than at present. Similar considerations apply to nearly all our geological measures of time; and there has been a tendency to exaggerate these, as if geologists were entitled to demand unlimited time, and to stretch the doctrine of uniformity to the utmost.
6. During the whole time referred to by geology, the great laws both of inorganic and organic nature have been the same as at present. The evidence of light and darkness, of sunshine and shower, of summer and winter, and of all the known igneous and aqueous causes of change, extends back almost, and in some of these cases altogether, to the beginning of the Palæozoic period. In like manner the animals and plants of the oldest rocks are constructed on the same physiological and anatomical principles with existing tribes, and they can be arranged in the same genera, orders, or classes, though specifically distinct. The revolutions of the globe have involved no change of the general laws of matter; and though it is possible that geology has carried us back to the time when the laws that regulate life began to operate, it does not show that they were less perfect than now, and it indicates no trace of the beginning of the inorganic laws. Geological changes have resulted not from the institution of new laws, but from new dispositions, under existing laws and general arrangements. There is every reason to believe that in the inorganic world these dispositions have required no new creative interpositions during the time to which geology refers, but merely the continued action of the properties bestowed on matter when first produced. In the organic world the case is different.
7. In the succession of animal and vegetable life we find a constant improvement and advance by the introduction of new types of being. We have already given a general outline of this advancement of organized nature. It has consisted in the introduction, from time to time, of new and more highly organized beings, so as at once to increase the variety of nature, and to provide for the elevation of the summit of the graduated scale of life to higher and higher points. At the same time, in each successive period, it has been the law of creation that the forms of life then dominant should attain their highest development, and should then be succeeded by more advanced types. For instance, in the earlier Palæozoic period we have molluscous animals and fishes, then apparently the highest forms of life, appearing with a very advanced organization, not surpassed, if even equalled, in modern times. In the latter part of the same period, some lower forms of vegetable life, now restricted to a comparatively humble place, were employed to constitute magnificent forests. In the Mesozoic period, again, reptiles attained to their highest point in organization and variety of form and employment, while mammalia had as yet scarcely appeared. [146]
8. If now we ask in what manner the succession of life on the earth has been produced, two apparently opposite hypotheses rise before us. The one is that of introduction of new species by creative acts, the other that of development of new species by changes of those previously existing. In one respect the difference of these views is little more than one of expression, for the meaning of the statements depends on what we understand by a species and what by a mere varietal form, and also on what we understand by creation and what we mean by development. Twenty years ago nearly all geologists were believers in creation, though it must be admitted without precisely understanding what they meant by the term. Now, the great impression produced by Darwin's speculations and the prevalence of the evolutionist philosophy have produced a leaning in the other direction. More recently, however, the absurdities into which the extreme evolutionists find themselves driven have produced a reaction; and we hope that views consistent with revelation, or at least with Theism, will again be in the ascendant, and that present controversies will serve to give more precise and definite views than heretofore of the relation of nature to God. As illustrations of the opinions prevalent before the rise of the development theory, I may quote from Pictet and Bronn, two of the most eminent palæontologists.
Pictet says, in the introduction to his "Traité de Paléontologie:" "It seems to me impossible that we should admit, as an explanation of the phenomena of successive faunas, the passage of species into one another; the limits of such transitions of species, even supposing that the lapse of a vast period of time may have given them a character of reality much greater than that which the study of existing nature leads us to suppose, are still infinitely within those differences which distinguish two successive faunas. Lastly, we can least of all account by this theory for the appearance of new types, to explain the introduction of which we must necessarily, in the present state of science, recur to the idea of distinct creations posterior to the first."
The following are the general conclusions of Bronn, in his elaborate and most valuable essay, presented to the French Academy in 1856, as summarized in a notice of the work in the Journal of the Geological Society: