has been to carry many groups back to a much earlier period in geological time. Twenty-five years ago we knew no insects earlier than the Devonian, no fishes earlier than the Ludlow beds just on the boundary between Silurian and Devonian.

Lyell's "Antiquity of Man" was published in 1863, and in 1867 the question of the time of man's origin was one of the burning questions. The co-existence of man with the mammoth and cave bear was not yet generally admitted, though it was a score of years or more since the beginning of Boucher de Perthes's discoveries in the gravels of the Somme Valley, and though Schmerling's investigations of the Belgian caves were of still earlier date. The man who doubts to-day the co-existence of man with the mammoth and the cave bear, is not an antagonist to be argued with, but an ignoramus to be sent to school; and certainly no scientist at present would dream of limiting the age of man to anything like the six thousand years of tradition. There is, nevertheless, at present a decided tendency to recede from Lyell's extreme claims in regard to the date of the origin of man. Some of the animals now extinct may have survived to a later date than was formerly sup posed.

The two volumes of the tenth edition of Lyell's "Principles of Geology" appeared respectively in 1867 and 1868. The discussion of the Darwinian theory contained in the second volume was therefore not before the public at the date from which we have commenced our survey, though that discussion had been anticipated by a somewhat more qualified statement of the same views in the work on the "Antiquity of Man."

But Lyell was in truth the forerunner of Darwin; and would have been no less so, even if he had failed himself to recognize his mission, and to welcome the new views for which he had done so much to prepare men's minds. The

triumph of Uniformitarianism over Catastrophism in the explanation of the inorganic history of the globe prepared the way for a belief in organic Evolution, since no Catastrophism could be so catastrophic as the notion of the origin of species by direct creation.

The extreme Uniformitarianism of Lyell not only favored the theory of Evolution, by its general principle of admitting no causes as having acted in the past save such as could be traced in the present; but favored especially a purely Darwinian form of the theory of Evolution, by making geological time extremely long. It may be that what most of us are now disposed to regard as the error of Lyell's views was indirectly favorable to the progress of truth, by making some of the geological difficulties in the way of the origin of species by natural selection seem less than they really were. Most of us at present are disposed vastly to reduce the limitless æons of geological time demanded by the Lyellian Geology of twenty-five years ago. Discussing, in 1867, Croll's theory of the cause of the glacial period, Lyell held that, if the glacial period was caused by an epoch of extreme eccentricity in the earth's orbit, it must have occurred in the epoch of great eccentricity which culminated about 850,000 years ago, rather than in that which culminated about 210,000 years ago. The improbability of the glacial epoch having been as ancient as even the later of those two epochs of high eccentricity is to most geologists to-day a strong reason for the rejection of Croll's theory.

The movement of the human mind in the progress of science is by no means a continuous and steady advance towards the truth. Theories may for considerable periods of time seem to be growing increasingly probable, and yet may ultimately be disproved. When the individual mind, or the collective mind of humanity, gets hold of a new idea, it tends to run to extremes. Inertia belongs to bodies in motion, as truly as to bodies at rest; and the mind possessed VOL. L. NO. 197.

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of a new idea displays an inertia of progress as irrational as the inertia of conservatism. The mind swings like a pendulum from one extreme to another, and in some cases a number of oscillations are required before a permanent resting-place is found. These oscillations of opinion are more conspicuous in some domains of thought than in others, being more strongly developed in proportion as observation is qualitative rather than quantitative, and in proportion as reasoning is inductive rather than demonstrative. We have already noticed one remarkable oscillation of this sort in biological science. Twenty-five years ago the doctrine of Spontaneous Generation seemed to be growing in favor, under the influence of the general tendency to find an evolutionary explanation for every class of phenomena. But the tide which then seemed flowing, has completely ebbed. The history of Geology in the last quarter-century presents a number of interesting examples of this oscillatory movement. Possibly the stimulation of the imagination by the contemplation of immeasurable magnitudes of forces and of times may render the opinions of geologists liable to this oscillatory movement in somewhat greater degree than those of some of their scientific brethren.

Two such examples of oscillation in geological opinion. have already been mentioned. Twenty-five years ago the extreme Uniformitarianism of Lyell, with its slow processes and immeasurable times, was thoroughly in the ascendant. From that position we have certainly receded. Geological opinion to-day is neither Uniformitarian nor Catastrophist. Croll's eccentricity theory of the glacial period, proposed in 1864, seemed in 1867 decidedly coming into favor. Lyell, in his tenth edition, treated it with decided approval, though not committing himself to its acceptance. The theory was a fascinating one, as linking in a new and interesting relation the sciences of Astronomy and Geology, and as giving to Geology somewhat of the chronological definiteness which

had belonged exclusively to its sister science. The theory, moreover, was immensely convenient, for we had not then, as, alas! we have not now, any thoroughly satisfactory geographical theory to account for the tremendous climatic mutations of Cenozoic time. But the tide seems to have turned, partly because, as already indicated, geological evidence shows that the glacial period was more recent than the latest epoch of high eccentricity; partly because it is very doubtful whether a condition of high eccentricity would tend to glaciation. A high eccentricity affects both the relative duration and the intensity of the seasons; but, as regards glaciation, the two conditions thus established tend to antagonize each other. A long winter tends to glaciation of the hemisphere whose winter falls in aphelion, but the hot summer tends to counteract this effect. The cool summer would favor accumulation of ice in the hemisphere whose summer is in aphelion, but the shortness of the winter diminishes the amount of snow.

Lyell followed Hutton in making the changes of continent and ocean perfectly kaleidoscopic, only Lyell differed from his predecessor in making the movements of elevation and subsidence involved in the change prodigiously slow. The reader of Darwin's "Letters" will remember his halfcomic, half-pathetic protest, in a letter to Lyell, that the great geologist and his "disciples in a slow and creeping manner beat all the old catastrophists who ever lived." Twenty-five years ago, this doctrine of the kaleidoscopic interchange of continent and ocean was probably accepted by the majority of geologists. It finds expression in those lines of Tennyson

"There rolls the deep where grew the tree.

O earth! what changes hast thou seen!

Where now the long street roars, hath been

The stillness of the central sea."

A score of years earlier, in 1846, Dana had announced the

opposite doctrine of the permanence of continent and ocean; and he has lived to see the opinion which he advocated in the beginning of his geological career win its way to almost universal adoption. Apart from any theoretical views on the still doubtful subject of the mode of the earth's cooling, and the constitution of the earth's interior, the doctrine of the permanence of continents and oceans is supported by three important lines of evidence. The sedimentary formations which cover the greater part of the continental areas, have, almost without exception, originated in shallow waters, even chalk being no longer supposed to have been deposited in oceanic depths. The oceanic islands are nearly all volcanic. The boundaries of the range of various groups of plants and animals coincide with the present areas of deep sea. But apparently the pendulum has swung too far in this direction, and we must go back part of the way toward Lyell's views. There appear to be exceptions to all the generalizations which have been cited as favoring the doctrine of permanence of continents and oceans. Some stratified formations have been formed in deep water; some oceanic islands are not volcanic; and, while the present areas of deep sea form the boundaries of the range of many groups of organisms, and particularly of those groups which belong chiefly or exclusively to Cenozoic and recent times, the same boundaries do not hold for the organisms of earlier geological periods, or even for the existing representatives of groups of ancient origin. The distribution of the Glossopteris flora, in the Carboniferous period, in South Africa, India, and Australia, indicates an arrangement of continents and oceans widely different from that which now exists, and the hypothesis of Gondwana-land seems likely to come into general acceptance. It seems likely that our ultimate resting-place on the question of the permanence of continent and ocean will be somewhat intermediate between the posi