F.—BEARING OF GLACIAL PERIODS UPON THE INTERPRETATION OF GENESIS.
Whatever views may be taken as to that period of cold which occurs at the close of the Tertiary and beginning of the Modern period, it can not be held to have constituted any such break as to be considered, as it was at one time, an equivalent for the Biblical chaos. This is proved by the survival through this period of a very large proportion of the animals and plants still existing in the northern hemisphere. The chronological system of animals and plants has been continuous, as the Bible represents it, since their first appearance on earth.
It is further remarkable that while there is geological evidence of climates colder than the present in the temperate regions, there is equally good proof of warmer climates even within the arctic circle than those of the cold temperate regions at present. It is difficult to account for these vicissitudes of climate, and much controversy exists on the subject; but it seems certain that in the earlier Tertiary and Cretaceous periods, for example, the supplies of heat and light were so diffused over the earth as to permit the growth of a temperate vegetation in Greenland, and even in Spitzbergen. Geologists, however unwillingly, have been obliged to admit this as one of those great possibilities, altogether unexpected beforehand, which have been developed in the history of our planet. Various modes of explaining this succession of cold and warm periods have been adopted, all more or less hypothetical. Lyell has argued that it may be explained by a different distribution of land and water and of the ocean currents. Croll accounts for it by the varying eccentricity of the earth's orbit, in connection with the precession of the equinoxes. Evans by a shifting of the axis of rotation of the earth. Drayson, Bell, Warring, and others, by a change in the inclination of the earth's axis. Others by the secular diminution of the internal heat of the earth, and of that of the sun. Others by the supposed recurrence of periods in which the sun gives more or less heat, or in which the earth is passing through colder or warmer regions of space. As the subject is of interest with reference to possible correspondences of these great summers and winters of the earth with the stages of the creative work, it may be well to notice shortly the relative merits of these theories.
(1.) The hypothesis of Croll is one of the most ingenious and elaborate of the whole; but it has two great defects. One is that the causes alleged are so uncertain and so complicated that it is difficult to estimate their real value. Another is that it proves too much, namely, a regular succession of cold and warm periods throughout geological time, of which we have no good evidence, and which is on many grounds improbable.
(2.) That the earth's axis of rotation has continued unchanged throughout the whole of the geological ages seems proved by the fact that the principal lines of crumpling and upheaval from the Laurentian period downward are arranged in great circles of the earth tangent to the polar circle; and that the lines of deposit of sediment in the Palæozoic age are coincident with the present direction of the arctic currents.
(3.) Astronomers consider it improbable that the obliquity of the ecliptic has materially changed, and serious differences of opinion exist as to the effects which a greater or less obliquity would produce on climate. It seems certain, however, that a less obliquity would occasion a more uniform distribution of heat and light throughout the year; and this, co-operating with other causes leading to a warm climate, might enable a temperate vegetation to approach the pole more closely than at present.
(4.) That the energy of the sun's radiation and the internal heat of the earth have been slowly decreasing seems certain; but it is now generally admitted that these changes are so gradual that little effect can have been produced by them, except in the older geological periods, and that they can have no connection with the great glacial period of the Post-pliocene.
(5.) It is otherwise with the hypothesis that the sun's heat may, like that of some variable stars, have increased and diminished. There is, of course, no direct evidence of this, except the small differences observed in cycles of eleven and fifty-five years from the greater or less development of sunspots, and the analogy of observed variable stars. Still it is a possible cause of variations of climate. It might also aid in accounting for the extraordinary evidences of desert conditions and desiccation presented by the salt deposits of different geological periods in temperate latitudes.
(6.) The theory of the passage of the earth through zones of space of variable temperature is now generally abandoned, as there seems no reason to believe that such differences exist.
(7.) The theory of Lyell that changes in the distribution of land and water may, with the possible co-operation of other causes, have produced the observed diversities of climate, is that which seems best to meet the conditions presented. It is based on the known properties of land and water as to the absorption, radiation, and convection of heat, and on the remarkable diversities of climate in similar latitudes arising from this cause at present. Farther, it accords with the known fact that very great changes of level have occurred in connection with the glacial period. This theory undoubtedly embraces a true cause, admitted by all geologists, and it dispenses with the necessity of believing in the recurrence of glacial periods at regular intervals. It farther accords best with the evidence afforded by fossils, and especially by fossil plants. It has also the merit of directing due attention to the diversities of geographical conditions at different periods, and of dealing with causes of change operating within the earth itself. The only doubt with respect to it is its sufficiency to explain the changes which have occurred, and the view entertained of this will depend very much on the interpretation of the facts as to the intensity of the last glacial period. If moderate views can be taken of this, and if means can be found, by a less obliquity of the ecliptic or otherwise, to furnish a continuous supply of light in the arctic regions, the difficulties which have been alleged against it would disappear.
(8.) In connection with former periods of cold and warmth, and with the existence of temperate and tropical vegetation in polar latitudes, we should not forget that view which takes into account the probable effects of different conditions of the atmosphere, and the greater quantity of carbonic acid present in it, in early geological periods. This would, of course, best apply to the palæozoic floras, in so far as our present knowledge extends; but there may have been similar conditions in later periods. Dr. Sterry Hunt thus states this hypothesis:
"The agency of plants in purifying the primitive atmosphere was long since pointed out by Brongniart, and our great stores of fossil fuel have been derived from the decomposition, by the ancient vegetation, of the excess of carbonic acid of the early atmosphere, which through this agency was exchanged for oxygen gas. In this connection the vegetation of former periods presents the curious phenomenon of plants allied to those now growing beneath the tropics flourishing within the polar circles. Many ingenious hypotheses have been proposed to account for the warmer climate of earlier times, but are at best unsatisfactory, and it appears to me that the true solution of the problem may be found in the constitution of the early atmosphere, when considered in the light of Dr. Tyndall's beautiful researches on radiant heat. He has found that the presence of a few hundredths of carbonic-acid gas in the atmosphere, while offering almost no obstacle to the passage of the solar rays, would suffice to prevent almost entirely the loss by radiation of obscure heat, so that the surface of the land beneath such an atmosphere would become like a vast orchard-house, in which the conditions of climate necessary to a luxuriant vegetation would be extended even to the polar regions."
It is obvious that, in the production of complex effects of this kind, various causes, whether astronomical or connected with the mutations of the earth's crust, may have co-operated, and probably in all extreme cases did co-operate.
In any case it is evident that the vicissitudes of climate and the great pulsations of the crust, which have raised and depressed portions of the surface and changed the position of its covering of waters, have been potent agents in the hands of the Creator in effecting the changes and succession of living beings, which are thus, as Genesis intimates, children of the waters and of the land, and of the influences of the heavens. It is also interesting in this connection to observe that the occurrence of such periods of general warm climate as that in the Miocene shows that it would have been possible for man, under certain conditions, to have extended himself far more widely in his Edenic state than we can conceive of in the present condition of the earth. The modern world is perhaps even in this way "cursed" for man's sake