320 



NATURE 



[August 4, 1S92 



done, that in the visible structure of the earth itself no trace can 

 be found of the beginning of things, and that the oldest terrestrial 

 records reveal no physical conditions essentially different from 

 those in which we still live. They doubtless listened with 

 interest to the speculations of Kant, Laplace, and Herschel, on 

 the probable evolution of nebulae, suns, and planets, but it was 

 with the languid interest attaching to ideas that lay outside of 

 their own domain of research. They recognized no practical 

 connection between such speculations and the data furnished by 

 the earth itself as to its own history and progress. 



This curious lethargy with respect to theory on the part of 

 men who were popularly regarded as among the most speculative 

 followers of science would probably not have been speedily dis- 

 pelled by any discovery made within their own field of observa- 

 tion. Even now, after many years of the most diligent research, 

 the first chapters of our planet's history remain undiscovered or 

 undecipherable. On the great terrestrial palimpsest the earliest 

 inscriptions seem to have been hopelessly effaced by those of 

 later ages. But the question of the primeval condition and 

 subsequent history of the planet might be considered from the 

 side of astronomy and physics. And it was by investigations of 

 this nature that the geological torpor was eventually dissipated. 

 To our illustrious former President, Lord Kelvin, who occupied 

 this chair when the Association last met in Edinburgh, is mainly 

 due the rousing of attention to this subject. By the most 

 convincing arguments he showed how impossible it was to believe 

 in the extreme doctrine of uniformitarianism. And though, 

 owing to uncertainty in regard to some of the data, wide limits 

 of time were postulated by him, he insisted that within these 

 limits the whole evolution of the earth and its inhabitants must 

 have been comprised. While, therefore, the geological doctrine 

 that the present order of Nature must be our guide to the inter- 

 pretation of the past remained as true and fruitful as ever, it had 

 now to be widened by the reception of evidence furnished by a 

 study of the earth as a planetary body. The secular loss of 

 heat, which demonstrably takes place both from the earth and 

 the sun, made it quite certain that the present could not have 

 been the original condition of the system. This diminution of 

 temperature with all its consequences is not a mere matter of 

 speculation, but a physical fact of the present time as much as 

 any of the familiar physical agencies that affect the surface of 

 the globe. It points with unmistakable directness to that 

 beginning of things of which Hutton and his followers could 

 find no sign. 



Another modification or enlargement of the uniformitarian 

 doctrine was brought about by continued investigation of the 

 terrestrial crust and consequent increase of knowledge respecting 

 the history of the earth. Though Hutton and Playfair believed 

 in periodical catastrophes, and indeed required these to recur 

 in order to renew and preserve the habitable condition of our 

 planet, their successors gradually came to view with repugnance 

 any appeal to abnormal, and especially to violent manifestations 

 of terrestrial vigour, and even persuaded themselves that such 

 slow and comparatively feeble action as had been witnessed by 

 man could alone be recognized in the evidence from which 

 geological history must be compiled. Well do I remember in 

 my own boyhood what a cardinal article of faith this preposses- 

 sion had become. We were taught by our great and honoured 

 master, Lyell, to believe implicitly in gentle and uniform opera- 

 tions, extended over indefinite periods of time, though possibly 

 some, with the zeal of partisan'^, carried this belief to an ex- 

 treme which Lyell himself did not approve. The most stupen- 

 dous marks of terrestrial disturbance, such as the structure of 

 great mountain chains, were deemed to be more satisfactorily 

 accounted for by slow movements prolonged through indefinite 

 ages than by any sudden convulsion. 



What the more extreme members of the uniformitarian school 

 failed to perceive was the absence of all evidence that terrestrial 

 catastrophes even on a colossal scale might not be a part of the 

 present economy of this globe. Such occurrences might never 

 seriously affect the whole earth at one time, and might return 

 at such wide intervals that no example of them has yet been 

 chronicled by man. But that they have occurred again and 

 again, and even within comparatively recent geological times, 

 hardly admits of serious doubt. How far at different epochs 

 and in various degrees they may have included the operation of 

 cosmical influences lying wholly outside the planet, and how 

 far they have resulted from movements within the body of the 

 planet itself, must remain for further inquiry. Yet the admis- 



NO. 1 188, VOL. 46] 



sion that they have played a part in geological history may be 

 freely made without impairing the real value of ihe Huttonian 

 doctrine, that in the interpretation of this history our main 

 must be a knowledge of the existing processes of terrestrial change. 



As the most recent and best known of these great transforma- 

 tions, the Ice Age stands out conspicuously before us. If any 

 one sixty years ago had ventured to affirm that at no very 

 distant date the snows and glaciers of the Arctic regions 

 stretched southwards into France, he would have been treated 

 as a mere visionary theorist. Many of the facts to which he 

 would have appealed in support of his statement were already 

 well known, but they had received various other interpretations. 

 By some observers, notably by Hutton's friend, Sir James Hall, 

 they were believed to be due to violent debacles of water that 

 swept over the face of the land. By others they were attributed 

 to the strong tides and currents of the sea when the land stood 

 at a lower level. The uniformitarian school of Lyell had no 

 difficulty in elevating or depressing land to any required extent. 

 Indeed, when we consider how averse these philosophers were 

 to admit any kind or degree of natural operation other than 

 those of which there was some human experience, we may well 

 wonder at the boldness with which, on sometimes the slenderest 

 evidence, they made land and sea change places, on the one 

 hand submerging mountain-ranges, and on the other placing 

 great barriers of land where a deep ocean rolls. They took 

 such liberties with geography because only well-established 

 processes of change were invoked in the operations. Knowing 

 that during the passage of an earthquake a territory bordering 

 the sea may be upraised or sunk a i&^ feet, they drew the 

 sweeping inference that any amount of upheaval or depression 

 of any part of the earth's surface might be claimed in explana- 

 tion of geological problems. The progress of inquiry, while it 

 has somewhat curtailed this geographical license, has now made 

 known in great detail the strange story of the Ice Age. 



There cannot be any doubt that afcer man had become a 

 denizen of the earth, a great physical change came over the 

 northern hemisphere. The climate, which had previously been 

 so mild that evergreen trees flourished within ten or twelve 

 degrees of the north pole, now became so severe that vast 

 sheets of snow and ice covered the liorth of Europe and crept 

 southward beyond the south coast of Ireland, almost as far as 

 the southern shores of England, and across the Baltic into 

 France and Germany. This Arctic transformation was not an 

 episode that lasted merely a few seasons, and left the land to 

 resume thereafter its ancient aspect. With various successive 

 fluctuations it must have endured for many thousands of years. 

 When it began to disappear it probably faded away as slowly 

 and imperceptibly as it had advanced, and when it finally 

 vanished it left Europe and North America profoundly changed 

 in the character alike of their scenery and of their inhabitants. 

 The rugged rocky contours of earlier times were ground smooth 

 and polished by the march of the ice across them, while the 

 lower grounds were buried under wide and thick sheets of clay, 

 gravel, and sand, left behind by the melting ice. The varied 

 and abundant flora which had spread so far within the Arctic 

 circle was driven away into more southern and less ungenial 

 climes. But most memorable of all was the extirpation of the 

 prominent large animals which, before the advent of the ice, 

 had roamed over Europe. The lions, hysenas, wild horses, 

 hippopotami, and other creatures either became entirely extinct 

 or were driven into the Mediterranean basin and into Africa. 

 In their place came northern forms — the reindeer, glutton, musk 

 ox, woolly rhinoceros, and mammoth. 



Such a marvellous transformation in climate, in scenery, in 

 vegetation and in inhabitants, within what was after all but a 

 brief portion of geological time, though it may have involved no 

 sudden or violent convulsion, is surely entitled to rank as a 

 catastrophe in the history of the globe. It was probably 

 brought about mainly if not entirely by the operation of forces 

 external to the earth. No similar calamity having befallen the 

 continents within the time during which man has been recording 

 his experience, the Ice Age might be cited as a contradiction to 

 the doctrine of uniformity, and yet it manifestly arrived as part 

 of the established order of Nature. Whether or not we grant 

 that other ice ages preceded the last great one, we must admit 

 that the conditions under which it arose, so far as we know 

 them, might conceivably have occurred before and may occur 

 again. The various agencies called into play by the extensive 

 refrigeration of the northern hemisphere were not different 

 from those with which we are familiar. ; Snow fell and glaciers 



