There is another and a very striking fact in the succession of ancient organic life, which claims from us a moment’s notice. As we proceed upward through the series of Stratified Rocks, from the oldest to the newest, we find a gradual advance in the types of animal organization therein preserved, from the humbler and more simple forms of structure to those of a higher and more perfect character. That form of organization is regarded among Zoologists as the more perfect in which there is “a greater number of organs specially devoted to particular functions.” Now all the forms of animal life with which we are acquainted, may be reduced to two great divisions, the Vertebrate and the Invertebrate,—the former having a vertebral or spinal column, the latter having none: and it is agreed in conformity with the notion set forth above, that the Vertebrate animals as a class exhibit a more perfect organization than the Invertebrate. Again, among the Vertebrate themselves there is a gradation; the Reptiles are ranked higher than the Fish, the Birds higher than the Reptiles, and the Mammalia higher again than the Birds.
All this we learn from Zoologists, who have pursued their investigations without any reference whatever to the science of Geology. It is, therefore, not a little remarkable that we should discover this very order and gradation of animal life in the successive groups of Stratified Rocks. All the Remains hitherto discovered in the earliest Geological Formations belong to Invertebrate animals, while the Vertebrate, which appear for the first time in the latter part of the Silurian Period, are, from that age on, more and more fully developed down to the present day, and now constitute, if not the most numerous, at least the most important part of the animal creation. Moreover, it is to be observed that the Vertebrate animals do not all make their appearance at once, but come in successively according to the same scale of organic perfection,—the Fish appearing first, then the Reptiles, then the Birds, and lastly the Mammalia. Even among the Mammalia a well-defined order of progressive succession has been observed, which finally culminates in the appearance of Man, the last created and the most perfect of animals.
TABLE OF GEOLOGICAL FORMATIONS,
SHOWING THE FIRST APPEARANCE ON THE EARTH OF THE VARIOUS FORMS OF ANIMAL LIFE.
This remarkable succession of animal life in the history of the Earth’s Crust will be more readily understood by means of the annexed Table. The remains of Invertebrate animals have been traced as far back as the Lower Laurentian Rocks. The Vertebrate first become manifest in the Ludlow beds of the Upper Silurian; where they are represented by the bones of Fish, the lowest class belonging to the Province of Vertebrates. Next in order come the Reptiles: the oldest known Reptile having been found in the Coal Measures of Saarbrück between Strasburg and Treves. The skeletons of Birds are rare in the Stratified Rocks. It is supposed that their powers of flight have in all ages secured them, to great a extent, from being carried away by floods, like other land animals, and buried in the sedimentary deposits of rivers and estuaries. Nevertheless their presence in the ancient world is frequently attested by their footsteps, impressed originally on the sandy beach, and still preserved now that the soft sand has been converted into solid rock. Such traces have been discovered in great abundance on the New Red Sandstone of the Connecticut River in America; and afford the earliest evidence we possess in the records of Geology regarding the existence of the feathered tribe. This group of strata belongs to the lower Trias. In the higher beds of the same Formation we meet with the first relic of ancient Mammals. It was found near Stuttgardt, in 1847, and belongs to the more imperfect form of Mammalian life, the Non-Placental. Similar remains have been since discovered in the Upper Trias of Somersetshire. The Placental, or more perfect form of animal life in the same class, first appears in the Eocene Formation: and the bones of Man, the highest of the Placental, are found for the first time in the upper deposits of the Post-Tertiary Age.
Let it be remembered that we are here but stating the facts which have been hitherto brought to light by the researches of Geologists. It may be, it is indeed most probable, that new discoveries will lead to numerous modifications in our Table. There is no reason to suppose that Geologists have yet exhumed the earliest remains of Vertebrates or Invertebrates preserved in the Crust of the Earth: that Fish may not hereafter be traced back beyond the Silurian, or Reptiles beyond the Carboniferous Period: that Birds may not be found among the Primary Rocks, and Placentals among the Secondary. But in a science which depends mainly upon observation, it is better to register the facts we have than to speculate idly about those we have not. And, having registered them, we cannot fail to be struck with the succession of animal life on the Earth, to which they seem to point. It is certainly deserving of notice that, as far as the Organic Remains hitherto discovered may be taken as a guide, Invertebrates and Vertebrates, Fish, Reptiles, Birds, and Mammals, Non-Placentals and Placentals, follow one another in the ascending series of Geological Formations exactly in the same order as they follow one another in the ascending scale of Zoological Classification.
And so Geologists go on ever searching out new phenomena, and grouping them together into classes, until from particular facts they lead us to general truths. Then starting with these general truths as the groundwork of their science, they proceed to sketch out the Natural History of our Globe from the remotest ages of the past down to the present time. They first study the stratified deposits of each succeeding age, and analyze the Fossil Remains embedded therein; afterward they make their inferences, and they compile their history. They describe the forms, the character, the habits, of the plants and animals that flourished of old in this world of ours; they tell us where the deep sea rolled its waves in each succeeding age, and where the dry land appeared; they point out the Deltas of its ancient rivers, they measure the breadth of its Estuaries, they trace the course of its Glaciers, they mark the outlines of its Mountain chains. But with these and such like speculations we are not here concerned. Many of them are open to controversy, and not a few are at this moment warmly disputed among Geologists themselves: besides, whether true or false, they do not in any way affect the relations between Geology and Revealed Religion. We shall be quite content, and it is all that our present scope demands, if we have made intelligible the general theory of Geological Chronology, and the kind of evidence on which it rests.
Before taking leave of this subject, however, we will venture to offer what seems to us an interesting illustration of the principles we have been explaining in the last two chapters;—one that will help to confirm the conclusions for which we have been contending, and that will also bring home to many minds the practical advantage to be derived from a thorough knowledge and just application of Geological science. Perhaps, too, it may help to revive the flagging attention of our readers; for the subject of our illustration is Coal, and the way to find it. In this age of manufactories and steam-engines,—when the atmosphere of great towns is heavy with smoke, and the quiet solitude of the country is so rudely disturbed by the shrieking of the railway-whistle and the snorting of the sooty locomotive,—this black, dirty mineral has acquired a value and importance, which may succeed in rousing even the practical money-making man to pay some heed to the lessons of science.
Coal might have been produced in any Geological Period; and in point of fact, beds of coal have been discovered in many different Formations. But in England, and in Western Europe generally, it has been found by long experience that the Coal-beds of the Carboniferous Formation are more abundant, and of better quality, than those of any other. Indeed the beds of Coal that occur in other Formations are so thin, and of such inferior quality, that they cannot be worked with profit. It is therefore of the highest importance in the search for Coal, before going to the enormous expense of sinking deep shafts, to discover whether or no the rocks in which the search is to be made belong to the Carboniferous Period. In this matter the more practical man is often seriously at fault. Coal-bearing strata generally consist pretty largely of dark-colored clay, black shales, and similar deposits. This is a fact which, as it strikes the eye, is perfectly familiar to all who are engaged in the working of Coal mines. Hence it happens, not unfrequently, that the practical man, when he meets with strata of this kind, is apt at once to infer that Coal is near at hand. The Geologist, on the contrary, knows well that such strata are not peculiar to the Carboniferous rocks, but are often found in other Formations in which there is no Coal at all, or at least no Coal that will repay the expense of working; and therefore he will pronounce it most rash to undertake costly works on the strength of these appearances. He has learned, however, that there are certain species of animals and plants which are found in the Carboniferous rocks and in them alone; he will search for these in the strata which it is proposed to explore, and by their presence or their absence he will know whether the strata in question belong to the Carboniferous Formation or not.