29. And thus the uniformitarian is led to consider the facts which geology brings to light, as indicating a slow and almost imperceptible, but, upon the whole, constant series of changes, not only in the position of the earth's materials, but in its animal and vegetable population. Land becomes sea and sea becomes land; the beds of oceans are elevated into mountain regions, carrying with them the remains of their inhabitants; sheets of lava pour from volcanic vents and overwhelm the seats of life; and these, again, become fields of vegetation; or, it may be, descend to the depths of the sea, and are overgrown with groves of coral; lakes are filled with sediment, imbedding the remains of land animals, and form the museums of future zoologists; the deltas of mighty rivers become the centres of continents, and are excavated as coal-fields by men in remote ages. And yet all this time, so slow is the change, that man is unaware such changes are going on. He knows that the mountains of Scandinavia are rising out of the Baltic at the rate of a few feet in a century; he knows that the fertile slope of Etna has been growing for thousands of years by the addition of lava streams and parasitic volcanos; he knows that the delta of the Mississippi accumulates hundreds of miles of vegetable matter every generation; he knows that the shores of Europe are yielding to the sea; but all these appear to him minute items, not worth summing; infinitesimal quantities, which he cannot integrate. And so, in truth, they are, for him. His ephemeral existence does not allow him to form a just conception, in any ordinary state of mind, of the effects of this constant agency of change, working through countless thousands of years. But Time, inexhausted and unremitting, sums the series, integrates the formula of change; and thus passes, with sure though noiseless progress, from one geological epoch to another.
30. And in the meanwhile, to complete the view thus taken by the uniformitarian of the geological history of the earth, by some constant but inscrutable law, creative agency is perpetually at work, to introduce, into this progressive system of things, new species of vegetable and animal life. Organic forms, ever and ever new ones, are brought into being, and left, visible footsteps, as it were, of the progress which Time has made;—marks placed between the rocky leaves of the book of creation; by which man, when his time comes, may turn back and read the past history of his habitation. But the point for us to remark is, the immeasurable, the inconceivable length of time, if any length of time could be inconceivable, which is required of our thoughts, by this new assumption of the constant production of new species, as a law of creation. We might feel ourselves well nigh overwhelmed, when, by looking at processes which we see producing only a few feet of height or breadth or depth during the life of man, we are called upon to imagine the construction of Alps and Andes,—when we have to imagine a world made a few inches in a century. But there, at least, we had something to start from: the element of change was small, but there was an element of change: we had to expand, but we had not to originate. But in conceiving that all the myriads of successive species, which we find in the earth's strata, have come into being by a law which is now operating, we have nothing to start from. We have seen, and know of, no such change; all sober and skilful naturalists reject it, as a fact not belonging to our time. We have here to build a theory without materials;—to sum a series of which every term, so far as we know, is nothing;—to introduce into our scientific reasonings an assumption contrary to all scientific knowledge.
31. This appears to me to be the real character of the assumption of the constant creation of new species. But, as I have said, it is not my business here, to pronounce upon the value or truth of this assumption. The only use which I wish to make of it is this:—If any persons, who have adopted the geological view which I have just been explaining, should feel any interest in the speculations here offered to their notice, they must needs be (as I have no doubt they will be) even more willing than other geologists, to grant to our argument a scale of time for geological succession, corresponding in magnitude to the scale of distances which astronomy teaches us, as those which measure the relation of the universe to the earth.
This being supposed to be granted, I am prepared to proceed with my argument.
CHAPTER VI.
THE ARGUMENT FROM GEOLOGY.
1. I have endeavored to explain that, according to the discoveries of geologists, the masses of which the surface of the earth is composed, exhibit indisputable evidence that, at different successive periods, the land and the waters which occupy it, have been inhabited by successive races of plants and animals; which, when taken in large groups, according to the ascending or descending order of the strata, consist of species different from those above and below them. Many of these groups of species are of forms so different from any living things which now exist, as to give to the life of those ancient periods an aspect strangely diverse from that which life now displays, and to transfer us, in thought, to a creation remote in its predominant forms from that among which we live. I have shown also, that the life and successive generations of these groups of species, and the events by which the rocks which contain these remains have been brought into their present situation and condition, must have occupied immense intervals of time;—intervals so large that they deserve to be compared, in their numerical expression, with the intervals of space which separate the planets and stars from each other. It has been seen, also, that the best geologists and natural historians have not been able to devise any hypothesis to account for the successive introduction of these new species into the earth's population; except the exercise of a series of acts of creation, by which they have been brought into being; either in groups at once, or in a perpetual succession of one or a few species, which the course of long intervals of time might accumulate into groups of species. It is true, that some speculators have held that by the agency of natural causes, such as operate upon organic forms, one species might be transmuted into another; external conditions of climate, food, and the like, being supposed to conspire with internal impulses and tendencies, so as to produce this effect. This supposition is, however, on a more exact examination of the laws of animal life, found to be destitute of proof; and the doctrine of the successive creation of species remains firmly established among geologists. That the extinction of species, and of groups of species, may be accounted for by natural causes, is a proposition much more plausible, and to a certain extent, probable; for we have good reason to believe that, even within the time of human history, some few species have ceased to exist upon the earth. But whether the extinction of such vast groups of species as the ancient strata present to our notice, can be accounted for in this way, at least without assuming the occurrence of great catastrophes, which must for a time, have destroyed all forms of life in the district in which they occurred, appears to be more doubtful. The decision of these questions, however, is not essential to our purpose. What is important is, that immense numbers of tribes of animals have tenanted the earth for countless ages, before the present state of things began to be.