XVI.

GEOLOGY AND EVOLUTION.

ALLUSION has frequently been made in preceding chapters to the fact that the deductions of geology and the theory of evolution possess many relations in common. The relationship in question has been demonstrated more than once in the course of our inquiries into the evidence which living nature presents of the truth of the development theory. For example, the case for "missing links," and for the substantiation or denial of the connected series of beings which evolution postulates, was seen to be one which must stand or fall accordingly as the geological evidence revealed the existence of "links" or not. If the history of the fossil contents of the rockformations could be shown to include no examples whatever of the "transitional forms" between species which the evolutionist demands, it is clear that the validity of his special conception of the order of nature would thereby be seriously impugned. The least cultured critic of the theory of descent would naturally turn to the geological evidence in search of confirmation or refutation of the views advanced by the followers of Mr. Darwin. Hence, in the early days of evolution, when the theory was in its infancy, the relations of geology to this conception of nature were foreseen to be of a kind demanding the most careful attention from both promulgators and opponents of the hypothesis of descent. In this view of matters, the chapters on the "Origin of Species," dealing with the "Imperfections of the Geological Record," and with the "Geological Succession of Organic Beings," have always been regarded with special interest alike by evolutionists and their opponents. Mr. Darwin had undoubtedly felt the high importance of the geological side of the great question he had propounded before the world of thought. The two chapters just mentioned, bear plain evidence of the painstaking care with which the venerable author of the classic work marshalled the evidence at hand, and with which also he reviewed the conclusions to which that evidence seemed to point. A perusal of the chapters in question will serve to show how consistently the facts of geology may be placed beside those revealed by the study of the life that exists to-day, and how the facts of life's past development often support, whilst they never negative, the ideas on which the evolutionist bases his belief.

The aspects which the geological evidence offers for the attention

of the evolutionist are manifold in number and variety. Our study of "missing links" has already shown us that one special phase of geological inquiry relates to the existence in the fossil record of those forms which may be described as intermediate in nature between existing species. If we suppose that no such forms had been known to exist, we can conceive that whilst their absence would not absolutely have negatived the theory of evolution, the want of such evidence would have decidedly weakened the evolutionist's case. The fulness with which the gaps have been supplied with even a relatively limited search in geological directions, has more than satisfied the evolutionist of the correctness of his deductions respecting the existence of the transitional forms he would expect the life of past æons to have exhibited. A second phase of geological inquiry in relation to the theory of descent is found in the question of past time and its duration. The objection has naturally been urged, that, if "natural selection" has operated in the past by the production of minute variations, acting through periods of immense antiquity, the drafts which the evolutionist is compelled to make on the bank of time are so great that collapse of the theory is the result. Physicists of the highest eminence have formulated opinions, based on apparently stable data, regarding the age of our planet. It has accordingly been urged that, assuming the most extended antiquity which such opinions allow, there yet remains a deficit in the age of the earth, compared with the demands or with the expectations of the evolutionist. But to all such speculations and considerations there remains one sufficient answer. So long as we are comparatively ignorant of the exact factors to which the work of evolution is due, it is idle to speculate on the time required for the change and modification of species. That specific change may have occasionally been more rapid in past ages than at present, is an idea for which there is considerable justification to be found in the history of the living beings we are able to study. Again, with imperfect knowledge of the progress and succession of climatal and other changes in the past, and with but shadowy ideas regarding the possibilities of biological change to-day, we have ample reason to regard the relations of time past to evolution, as by no means so simple as certain opinions would appear to indicate. The evidence of geology itself, whilst rejecting the ancient ideas of "catastrophism” and of sudden, cataclysmal changes, nevertheless must be held as proving that cosmical and physical actions have not always presented the phases we see exemplified before our eyes to-day. In a word, until we have at hand fuller evidence regarding the manifest interaction between cosmical alteration and biological change-between physical revolutions and their effect upon the life of our globe-all speculations concerning the duration of time past in its relation to

evolution must be held as purely tentative and provisional. The case for evolution, as indicated in our previous studies, is so overwhelmingly strong when we regard the evidence from biology alone, that the evolutionist may well be pardoned if he is inclined to turn a deaf ear to any arguments against his theory which are derived from data so manifestly imperfect in their details as those on which speculations concerning past time and life-development are based.

As bearing in the most intimate manner on this very question of time in relation to evolution, it may be interesting to point out that vast periods of unrepresented time must be allowed for in all considerations connected with the past history of the earth. If we tabulate the various fossil-bearing rock-formations in the order in which they occur in nature, the following table will represent their

Devonian, or Old Red Sandstone.
Silurian.

Cambrian (and Huronian).
Laurentian.

This table shows us that the fossil-bearing rocks are arranged in a very definite order and succession; the lowest, and therefore the oldest, series being the "Paleozoic" rocks, which in turn possess the Laurentian group as their most ancient formations, and the Permian rocks as their newest. Above these, again, lie the "Mesozoic rocks, with the Trias, lying above the Permian, as their oldest, and the Chalk, reposing in turn on the Oolite, as their youngest beds respectively. The "Kainozoic" rocks form the last and most recent series of all. They lie upon the Chalk, their oldest strata being those named the Eocene, and their youngest formations the soils and gravels of to-day. It is needless to remark that this tabular order is never anywhere seen in its entirety. Geological revolution disturbing the strata, has produced many and serious breaks in their continuity. But such gaps do not affect the order of their succession tabulated by the geologist; that is to say, the Permian, for example, wherever found, must overlie the Coal, as the latter in turn invariably lies above the Devonian. The complete thickness of the stratified rocks alone-or, in other words, their thickness exclusive

of volcanic or igneous rocks-in Britain is given by Professor Ramsay as 72,584 feet, or, as Mr. Darwin has put it, "very nearly thirteen and three-quarters British miles. Some of the formations," continues Mr. Darwin, "which are represented in England by these beds, are thousands of feet in thickness on the continent. Moreover, between each successive formation we have, in the opinion of most geologists, blank periods of enormous length."

Now, it is these "blank periods" which certainly affect, in the plainest fashion, all questions concerning the operation of biological change in its relations to time past. It is necessary here to bear in mind a few elementary geological axioms, such as the fact that only those rocks (the aqueous or stratified rocks) which have been formed in water contain fossils; and that the igneous, or volcanic, rocks cannot be expected, from the mere fashion of their formation, to present any traces of past life. Thus it is clear the aqueous, or fossil-bearing rocks must have been formed either in the beds of oceans or in shallow water along coast lines, or in lakes, or at the mouths of rivers, the materials for these rocks being supplied by the wear and tear of previously existing formations. The nature of the included fossils is often the best guide to the exact site wherein the soft materials were deposited to form the strata of future epochs. Thus the presence of fossilised corals, star-fishes, and marine forms of fishes would indicate that the formations containing these fossils had been deposited in a sea-bed; just as the discovery of freshwater shells and plants in another series of strata would show that these latter beds were of lake origin; or, as a mixture of fresh-water and marine forms would suggest that the strata had been deposited in brackish water.

The distinctness of any two or more series of strata is inferred by the geologist when he discovers differences in their included fossils, whilst he also possesses a criterion of their distinctness in their mineral characteristics. Now, each group of stratified rocks is more or less clearly characterised by possessing certain characteristic fossils, which in some cases may be absolutely confined and limited to the one group of formations, never passing into any other series; or may, on the other hand, extend from one group through another series, or even through several successive formations. But we are accustomed to note that certain fossils specially characterise each formation, and are characteristic of that formation, even when they may have been slightly developed prior to its period, or have passed beyond it into the next epoch. The comparative distinctness of the fossils in the several formations was formerly explained on the assumption, that at the close of each period the forms of life dwelling therein were extinguished and killed off by some sudden catastrophe. The life of the succeeding period was supposed to be

BB

produced by the special creation of new species; these in turn becoming extinct at the close of their particular epoch. Creation and extinction, on this theory, were thus alternating processes; the death of the one set of organisms heralded the production of the new and independent forms. Such a hypothesis, tenable enough, of course, on the theory of "special creation," is diametrically opposed to that of evolution. But the case for the latter hypothesis was soon proved to be overwhelmingly strong when the facts relating to fossils were more fully investigated. Thus, when it is clear that all the forms represented as fossils in one series of rocks are not, as a rule, completely absent from the succeeding epoch, but are often found represented in the next period, the case for the "special creation" of each new series of fossils becomes materially weakened. Furthermore, just as lines of genetic relationship connect existing. animals and plants, so like relations may be traced between extinct and fossil forms. Hence the theory that the fossil animals and plants of each period must represent the more or less typical descendants of the life of the preceding epoch, at once rises into the domain of rational belief. If we can believe that each new period is thus peopled by the descendants of the preceding epoch, and by the actual survivals from that period, the case for evolution grows in strength; and this belief is exactly that which modern geological science esteems to have been the true cause of the succession of life through the changing æons of the past. "If the doctrine of evolution is sound," says Professor Huxley, "one of its immediate consequences clearly is, that the present distribution of life upon the globe is the product of two factors, the one being the distribution which obtained in the immediately preceding epoch, and the other the character and extent of the changes which have taken place in physical geography between the one epoch and the other; or, to put the matter in another way, the Fauna and Flora of any given area, in any given epoch, can consist only of such forms of life as are directly descended from those which constituted the Fauna and Flora of the same area in the immediately preceding epoch, unless the physical geography (under which I include climatal conditions) of the area has been so altered as to give rise to immigration of living forms from some other area."

The succession of life thus described offers convincing proofs of the correctness of the evolutionist's views, and these proofs will be presently considered. The nature and influence of the breaks or "blank periods" of Mr. Darwin, which exist between the series, remain, however, for primary notice. The geologist finds ample cause in his study of the rock-masses to assume that the periods of time which have elapsed between the end of one epoch and the beginning of the next, have been of immense duration. That these