No doubt most people feel justified in believing that the whole doctrine of evolution must stand or fall according to the cogency of the palæontological evidences. Plain common sense says that the owners of shelly or bony fragments found in the deeply-laid strata of the earth must have lived countless years ago, and if the evolutionist asserts that primitive organic forms of ancient times have produced changed descendants of later times, it would seem that fossil evidence would be supremely and overwhelmingly important. It is true, of course, that this evidence is peculiarly significant, because in some ways it is more direct than that of the other categories already outlined. But it must not be forgotten that the doctrine is already securely founded upon the basic principles of anatomy and embryology. Science must treat the data of this category by different methods and must view them in different ways. Therefore we are interested in palæontology because of the way it tells the story of evolution in its own words, and because we are justified in expecting that its account should include a description of some such order of events as that revealed by the developing embryos of modern organisms and that demonstrated by the comparative anatomy of the varied species of adult animals.
It is true that palæontology gives direct testimony about the evolutionary succession of animals in geologic time. But we now know that embryology is even more direct in its proof that organic transformation is natural and real; while at the same time there is a completeness in the full series of developmental stages connecting the one-celled egg with the adult creature that must be forever lacking in the case of the fossil sequence of species. If paragraphs and pages are missing from the brief embryonic recapitulation, whole chapters and volumes of the fossil series have been lost for all time. The investigators whose task it has been to decipher the story of the earth's evolution have had to meet numerous and exasperating difficulties which do not confront the embryologist and anatomist who study living materials. Nevertheless the library of palæontological documents is one which has been founded for over a century, and it has grown fast during recent decades, so that consistent accounts may now be read of the great changes in organic life as the earth has altered and grown older. And in all this record, there is not a single line or word of fact that contradicts evolution. What definite evidence there is tells uniformly in favor of the doctrine, for it is possible, in the first place, to work out the order of succession of many of the great groups of animals, and this order is found to be the same as that established by the other bodies of evidence. Secondly, some fossil groups are astonishingly complete, so that the ancient history of a form like the horse can be written with something approaching fullness. Finally, the remains of certain animals have been found so situated in geological ways, and so constructed anatomically, that the zoölogist is justified in denoting them "missing links," because they seem to have been intermediate between groups that have diverged so widely during recent epochs as to render their common ancestry scarcely credible.
With these general results in mind, we must now become acquainted with such subjects as the interpretation of fossils, the causes for the incompleteness of the series, the conditions for fossilization, the forces of geological nature, and other matters that make the fossils themselves intelligible as scientific evidence.
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Many views have been entertained regarding the actual nature of the relics of antiquity exhumed from the rocks or exposed upon the surface by the wear and tear of natural agencies. In earliest times such things were variously considered as curious freaks of geological formation, as sports of nature, or as the remains of the slain left upon the battle-ground of mythical Titans. Some of the Greeks supposed that fossils were parts of animals formed in the bowels of the earth by a process of spontaneous generation, which had died before they could make their way to the surface. They were sometimes described as the bones of creatures stranded upon the dry land by tidal waves, or by some such catastrophe as the traditional flood of the scriptures. In medieval times, and even in our own day, some people who have been opposed to the acceptance of any portion of the doctrine of evolution have actually defended the view that the things called fossils were never the shells or bones of animals living in bygone times, but that they only simulate such things and have been created as such together with the layers of rock from which they may have been taken. If we employed the same arguments in dealing with the broken fragments of vases and jewelry taken from the Egyptian tombs or from the buried ruins of Pompeii, we would have to believe that such pieces were created as fragments and that they were never portions of complete objects, just because no one alive to-day has ever seen the perfect vessel or bracelet fashioned so long ago. Common sense directs us to discard such a fantastic interpretation in favor of the view that fossils are what they seem to be—simply relics of creatures that lived when the earth was younger.
Until this common sense view was adopted there was no science of palæontology. Cuvier was the first great naturalist to devote particular attention to the mainly unrelated and unverified facts that had been discovered before his time. He was truly the originator of this branch of zoölogy, for he brought together the observations of earlier men and extended his own studies widely and surely, emphasizing particularly the necessity for noting carefully the geological situation of a fossil in rocks of an older or later period of formation. His great result was the demonstration that many groups of animals existed in earlier ages that seem to have no descendants of the same nature to-day, and also that many or most of our modern groups are not represented in the earliest formed sedimentary rocks, although these recent forms possess hard parts which would surely be present somewhere in these levels if the animals actually existed in those times. But the meaning of these facts escaped Cuvier's mind. He was a believer in special creation, like Linnæus and all but a few among his predecessors, and he explained the diversity of the faunas of different geological times in what seems to us a very simple and naïve way. In the beginning, he held, when the world was created, it was furnished with a complete set of animals and plants. Then some great upheaval of nature occurred which overwhelmed and destroyed all living creatures. The Creator then, in Cuvier's view, proceeded to construct a new series of animals and plants, which were not identical with those of the former time, but were created according to the same general working plans or architectural schemes employed before. Another cataclysm was supposed to have occurred, which destroyed the second series of organisms and laid a new covering of rocks over the earth's surface for a subsequent period of relative quiet; and so the process was continued. By this account, Cuvier endeavored to reconcile the doctrine of supernatural creation and intervention with the obvious facts that organisms have differed at various times in the earth's history. Although he saw that animals of successive periods displayed similar structures, like the skeleton of vertebrates, which testified to some connection, Cuvier could not bring himself to believe that this connection was a genealogical one.
Mainly through the influence of the renowned English man of science, Charles Lyell, the students of the earth came to the conclusion that its manifold structures had developed by a slow and orderly process that was entirely natural; for they found no evidence of any sudden and drastic world-wide remodeling such as that postulated by the Cuvierian hypothesis of catastrophe. The battle waged for many years; but now naturalists believe that the forces, of nature, whose workings may be seen on all sides at the present time, have reconstructed the continents and ocean beds in the past in the same way that they work to-day. The long name of "uniformitarianism" is given to Lyell's doctrine, which has exerted an influence upon knowledge far outside the department of geology. Darwin tells us how much he himself was impressed by it, and how it led him to study the factors at work upon organic things to see if he could discern evidence of a biological uniformitarianism, according to which the past history of living things might be interpreted through an understanding of their present lives.
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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.