The Ancient Life History of the Earth. Henry Alleyne Nicholson

what is now the European area, and to be peopled by certain species of animals. Beds of sediment—clay, sands, and limestones—will be deposited over the sea-bottom, and will entomb the remains of the animals as fossils. After this has lasted for a certain length of time, the European area may undergo elevation, or may become otherwise unsuitable for the perpetuation of its fauna; the result of which would be that some or all of the marine animals of the area would migrate to some more suitable region. Sediments would then be accumulated in the new area to which they had betaken themselves, and they would then appear, for the second time, as fossils in a set of beds widely separated from Europe. The second set of beds would, however, obviously not be strictly or literally contemporaneous with the first, but would be separated from them by the period of time required for the migration of the animals from the one area into the other. It is only in a wide and comprehensive sense that such strata can be said to be contemporaneous.

      It is impossible to enter further into this subject here; but it may be taken as certain that beds in widely remote geographical areas can only come to contain the same fossils by reason of a migration having taken place of the animals of the one area to the other. That such migrations can and do take place is quite certain, and this is a much more reasonable explanation of the observed facts than the hypothesis that in former periods the conditions of life were much more uniform than they are at present, and that, consequently, the same organisms were able to range over the entire globe at the same time. It need only be added, that taking the evidence of the present as explaining the phenomena of the past—the only safe method of reasoning in geological matters—we have abundant proof that deposits which are actually contemporaneous, in the strict sense of the term, do not contain the same fossils, if far removed from one another in point of distance. Thus, deposits of various kinds are now in process of formation in our existing seas, as, for example, in the Arctic Ocean, the Atlantic, and the Pacific, and many of these deposits are known to us by actual examination and observation with the sounding-lead and dredge. But it is hardly necessary to add that the animal remains contained in these deposits—the fossils of some future period—instead of being identical, are widely different from one another in their characters.

      We have seen, then, that the entire stratified series is capable of subdivision into a number of definite rock-groups or "formations," each possessing a peculiar and characteristic assemblage of fossils, representing the "life" of the "period" in which the formation was deposited. We have still to inquire shortly how it came to pass that two successive formations should thus be broadly distinguished by their life-forms, and why they should not rather possess at any rate a majority of identical fossils. It was originally supposed that this could be explained by the hypothesis that the close of each formation was accompanied by a general destruction of all the living beings of the period, and that the commencement of each new formation was signalised by the creation of a number of brand-new organisms, destined to figure as the characteristic fossils of the same. This theory, however, ignores the fact that each formation—as to which we have any sufficient evidence—contains a few, at least, of the life-forms which existed in the preceding period; and it invokes forces and processes of which we know nothing, and for the supposed action of which we cannot account. The problem is an undeniably difficult one, and it will not be possible here to give more than a mere outline of the modern views upon the subject. Without entering into the at present inscrutable question as to the manner in which new life-forms are introduced upon the earth, it may be stated that almost all modern geologists hold that the living beings of any given formation are in the main modified forms of others which have preceded them. It is not believed that any general or universal destruction of life took place at the termination of each geological period, or that a general introduction of new forms took place at the commencement of a new period. It is, on the contrary, believed that the animals and plants of any given period are for the most part (or exclusively) the lineal but modified descendants of the animals and plants of the immediately preceding period, and that some of them, at any rate, are continued into the next succeeding period, either unchanged, or so far altered as to appear as new species. To discuss these views in detail would lead us altogether too far, but there is one very obvious consideration which may advantageously receive some attention. It is obvious, namely, that the great discordance which is found to subsist between the animal life of any given formation and that of the next succeeding formation, and which no one denies, would be a fatal blow to the views just alluded to, unless admitting of some satisfactory explanation. Nor is this discordance one purely of life-forms, for there is often a physical break in the successions of strata as well. Let us therefore briefly consider how far these interruptions and breaks in the geological and palæontological record can be accounted for, and still allow us to believe in some theory of continuity as opposed to the doctrine of intermittent and occasional action.

       In the first place, it is perfectly clear that if we admit the conception above mentioned of a continuity of life from the Laurentian period to the present day, we could never prove our view to be correct, unless we could produce in evidence fossil examples of all the kinds of animals and plants that have lived and died during that period. In order to do this, we should require, to begin with, to have access to an absolutely unbroken and perfect succession of all the deposits which have ever been laid down since the beginning. If, however, we ask the physical geologist if he is in possession of any such uninterrupted series, he will at once answer in the negative. So far from the geological series being a perfect one, it is interrupted by numerous gaps of unknown length, many of which we can never expect to fill up. Nor are the proofs of this far to seek. Apart from the facts that we have hitherto examined only a limited portion of the dry land, that nearly two-thirds of the entire area of the globe is inaccessible to geological investigation in consequence of its being covered by the sea, that many deposits can be shown to have been more or less completely destroyed subsequent to their deposition, and that there may be many areas in which living beings exist where no rock is in process of formation, we have the broad fact that rock-deposition only goes on to any extent in water, and that the earth must have always consisted partly of dry land and partly of water—at any rate, so far as any period of which we have geological knowledge is concerned. There must, therefore, always have existed, at some part or another of the earth's surface, areas where no deposition of rock was going on, and the proof of this is to be found in the well-known phenomenon of "unconformability." Whenever, namely, deposition of sediment is continuously going on within the limits of a single ocean, the beds which are laid down succeed one another in uninterrupted and regular sequence. Such beds are said to be "conformable," and there are many rock-groups known where one may pass through fifteen or twenty thousand feet of strata without a break—indicating that the beds had been deposited in an area which remained continuously covered by the sea. On the other hand, we commonly find that there is no such regular succession when we pass from one great formation to another, but that, on the contrary, the younger formation rests "unconformably," as it is called, either upon the formation immediately preceding it in point of time, or upon some still older one. The essential physical feature of this unconformability is that the beds of the younger formation rest upon a worn and eroded surface formed by the beds of the older series (fig. 18); and a moment's consideration will show us what this indicates. It indicates, Fig. 18.—Section showing strata of Tertiary age (a) resting upon a worn and eroded surface of White Chalk (b), the stratification of which is marked by lines of flint. beyond the possibility of misconception, that there was an interval between the deposition of the older series and that of the newer series of strata; and that during this interval the older beds were raised above the sea-level, so as to form dry land, and were subsequently depressed again beneath the waters, to receive upon their worn and wasted upper surface the sediments of the later group. During the interval thus indicated, the deposition of rock must of necessity have been proceeding more or less actively in other areas. Every unconformity, therefore, indicates that at the spot where it occurs, a more or less extensive series of beds must be actually missing; and though we may sometimes be able to point to these missing strata in other areas, there yet remains a number of unconformities for which we cannot at present supply the deficiency even in a partial manner.

      It follows from the above that the series of stratified deposits is to a greater or less extent irremediably imperfect; and in this imperfection we have