[CHAPTER XII.]
PRE-DETERMINATION IN NATURE.
T
The natural prejudice of persons not acquainted with geology is that in the world all things continue as they were from the beginning. But a little observation and experience dispels this delusion, and perhaps replaces it with an opposite error. When our minds have been familiarized with the continuous processes by which vaporous nebulæ may be differentiated into distinct planets, and these may be slowly cooled from an incandescent state till their surfaces become resolved into areas of land and water; and still more, when we contemplate the grand procession of forms of life from the earliest animals and plants to man and his contemporaries, we become converts to the doctrine that all things are in a perpetual flux, and that every succeeding day sees them different from what they were the day before. In this state of mind the scientific student is apt to overlook the fact that there are many things which remain the same through all the ages, or which, once settled, admit of no change. I do not here refer to those fundamental properties of matter and forces and laws of nature which form the basis of uniformitarianism in geology, but to determinations and arrangements which might easily have been quite different from what they are, but which, once settled, seem to remain for ever.
We have already considered the great fact that the nuclei and ribs of the continental masses were laid down as foundations in the earliest periods, and have been built upon by determinate additions, more especially upon their edges and their hollows, so that while there has been a constant process of removal of material from the higher parts of the land, and deposition in the sea, and while there have been periodical elevations and subsidences, the great areas of land and water have remained substantially the same, and the main lines of elevation and folding have conformed to the directions originally fixed. Thus, in regard to the dry land itself, there has been fixity, on the one hand, and mutation on the other, of a most paradoxical aspect, till we understand something of the great law of constant change united with perennial fixity in nature. From want of attention to this, the permanence of continents is still a debated question, and it is difficult for many to understand how the frequent dips of the continental plateaus and margins under the sea, and their re-elevation, often along with portions of the shallower sea bottom, can be consistent with a general permanence of the position of the continents and of the corresponding ocean abysses; yet, when this is properly understood, it becomes plain that the union of fixity with changes of level has been a main cause of the continuity and changes of organic beings. Only the submergence of inland plateaus under shallow and warm waters could have given scope for the introduction of new marine faunas, and only re-elevation could have permitted the greatest extension of plants and animals of the land. Thus, the continuity of life with continual advance has depended on the permanent existence of continental and oceanic areas; and the continents that remain to us with all their diversity of elevation and outline, their varied productions, both mineral and organic, and their life, which is a select remainder of all that went before, have been produced and furnished by a succession of changes, modified by the most conservative retention of general arrangements and forms.
It is evident, however, that it is not merely permanence we have to deal with here, but permanence of position along with change of elevation; and this modified by the fact that there have always been mountain ridges, internal plateaus, and marginal areas affected in various ways by the vertical movement of the land. Further, the elevation and subsidence of the land have not always been uniform, but often differential, while every movement has tended to produce modifications of ocean currents and of atmospheric conditions. The whole subject, more especially in its relations to life, thus becomes very complicated, and it is perhaps in consequence of partial and imperfect views on these points that so much diversity of opinion has arisen. For example, it is evident that we can gain nothing by adding to the continents those submerged margins delineated by Murray in the Challenger reports, and which have in periods of continental elevation themselves formed portions of the land. Nor do we establish a case in favour of perished oceanic continents by the argument that they are needed to furnish the materials of marginal mountains which are due to the continuous sweeping of arctic material to the south by currents, as we see in the coast of North America to-day. Nor do we invalidate the permanence of the continents by the bridges of land, islands, and shallow water at various times thrown across the Atlantic. The distribution of Cambrian Trilobites, as illustrated by Matthew,[151] seems to show a bridge of this kind in the north in very early times, and similar evidence is furnished by the animals and plants of the Devonian and Carboniferous, and by the sea animals and plants of the later Tertiary and modern. Gardener has postulated a southern bridge in the region of the West Indies for the migrations of plants, and Gregory has adduced the evidence of those conservative and slow-moving creatures, the sea urchins, in favour of similar connection in the West Indian region at two distinct periods of time (the Lower Cretaceous and the Miocene Tertiary). But bridges do not involve want of permanence in their termini. Because an engineer has bridged the Firth of Forth, it does not follow that the banks of this inlet did not exist before the bridge was built; and if the bridge were to perish, the evidence that trains had once passed that way would not justify the belief that the bed of the Firth had been dry land, and the areas north and south of it depressed. The more we consider this question the more we see that the permanence, growth and sculpture of the continents are parts of a great continuous and far-reaching plan. This view is strengthened rather than otherwise, when we consider the probable manner in which the enormous weight of the continents is sustained above the waters. We may attribute this, on the one hand, to rigidity and lateral arching and compression, or, on the other, to what may be termed flotation of the lighter parts of the crust; and there seems to be little doubt that both of these principles have been employed in constructing the "pillars which support the earth." It is evident, however, that an arch thrown over the internal abyss of the earth, or a portion of its crust so lightened as to be pressed upward by its heavier surroundings, must, when once established, have become a permanent feature of the earth's foundations, not to be disturbed without calamitous consequences to its inhabitants.
[151] Transactions Royal Society of Canada, 1892.
It is the part of the philosophical naturalist to bring together these apparent contrarieties of mutation and permanence; both of which are included, each in its proper place, in the great plan of nature. It is therefore my purpose in the present chapter to direct attention to some of the terminal points or fixed arrangements that we meet with in the course of the geological history, and even in its earlier parts, and more particularly in reference to the organic world. This, which is in itself constantly changing, has been placed under necessity to adhere to certain determinations fixed of old, and which regulate its forms and possibilities down to our own time.