June;- 19, 1890] 



NATURE 



179 



untenable ; a depression of temperature is testified to, not 

 only by the extension of the glaciers, but also by the 

 vegetable and animal denizens of the land and the'sea. 

 When in Pleistocene deposits of the Mediterranean basin 

 we find Mollusca suddenly appear which now live only 

 in the German Ocean, no other explanation is possible 

 than that the temperature at that time was low. 



We need not indeed conclude that an excessive degree 

 of cold was necessary to produce the phenomena of the 

 Glacial period ; the height of the snow-line at that time 

 has been computed for many of the mountains of Europe, 

 and from this it has been deduced that the extreme reduc- 

 tion of temperature was at the utmost 6° C. (ii° F.), and 

 possibly considerably less. Much has been said and 

 written of the causes which brought about the cold of the 

 Glacial period. Very thoughtful and also very jejune 

 hypotheses have been put forward, all of which have this 

 one characteristic in common, that in some one particular 

 or another they are strongly opposed to the actual facts, 

 and have therefore no validity. With our present know- 

 ledge, any explanation is quite impossible. We must 

 content ourselves with recognizing that the cooling was 

 simultaneous, and, as far as research has yet gone, ex- 

 tended over the whole of the globe. It is, then, obviously 

 impossible to attribute it to a displacement of the pole, 

 for in that case a part of the earth must have experienced 

 an increase of temperature ; and, in addition to this, we 

 certainly cannot suppose any considerable change in the 

 position of the pole within so comparatively short an 

 interval as separates us from the Glacial epoch. The 

 uniform extension of the phenomenon excludes all those 

 attempted explanations which appeal to geological or 

 geographical changes of the earth's surface, a dififerent 

 distribution of land and sea, changes in the ocean currents, 

 &c., and all points to some agency external to the earth, 

 and therefore acting on it as a whole. 



We must specially notice one other circumstance in 

 connection with the Glacial period. It has been observed 

 in many places that the glacial deposits with their 

 scratched pebbles and irregular heaping of their materials 

 do not form a continuous mass, but that, between a lower 

 and upper deposit of glacial character, there is an inter- 

 mediate bed showing no trace of ice action ; at different 

 places, the remains of animals and plants have been met 

 with in this intermediate bed which indicate a somewhat 

 warmer climate, though slightly colder than the present. 

 Thus, in the slaty coal of Utznach and Diirnten in 

 Switzerland, which belongs to this formation, have been 

 found only the remains of plants still growing in the 

 neighbourhood, with the single exception of the mountain 

 pine, which no longer exists in the low plains of Switzer- 

 land, but has withdrawn to Alpine heights. These 

 so-called inter-glacial deposits attain in places to a con- 

 siderable thickness. They show us that during the great 

 Glacial period there intervened a very decided recurrence 

 of a warmer temperature, during which the great ice 

 masses melted away ; and from all the indications, 

 this interval, according to human reckoning, must have 

 lasted thousands of years. This page of the earth's history 

 has for us this especial interest, that the oldest certain 

 indications of man's existence in Europe are found in 

 these inter-glacial deposits. 



Similar evidence of an interruption of the Glacial period 

 by one of greater warmth is met with in many other parts 

 of the Alpine region, and also on the plains of Northern 

 Germany, in Scandinavia, England, and in different parts 

 of North America, and we must therefore conclude that 

 it was of general occurrence, and that the changes of 

 temperature which brought about the glaciation of an 

 enormous extent of land, and subsequently set it free 

 from its icy covering, were not regularly progressive, but 

 consisted of many changes and oscillations. . . . 



Thus we have sketched in a few hasty outlines what 

 we know of the climatic conditions of three periods 



NO. 1077, VOL. 42] 



of the earth's history which are of especial importance 

 forjudging such questions. The first of these, of hoar 

 antiquity, was that of the Coal-measures. We have 

 ascertained the existence of distinct floral regions, which 

 in all probability were determined by differences in the 

 distribution of heat ; moreover, we have found in deposits 

 far distant from each other evidence of ice action. But 

 in all other points the conditions are so far removed from 

 any of which we have experience, that any further in- 

 ference is hardly possible. At the utmost we may con- 

 clude from the limitation of the greater coal-beds to the 

 temperate zone that the position of the earth's axis and 

 of the pole did not differ very greatly from those of the 

 present day. 



When we turn to the much younger formations of the 

 Tertiary age, the conditions are somewhat clearer. In 

 them we recognize, in the first place, the operation of 

 purely local agencies, the distribution of land and water, 

 of ocean currents, &c., but we must also confess that these 

 play but a subordinate part. We have also seen that in 

 certain regions, viz., in Europe, Greenland, Grinnell Land, 

 &c., there prevailed a much warmer climate, which, how- 

 ever, we do not recognize in America ; while in Japan, as 

 inferred from the vegetation, the temperature of Tertiary 

 times seems to have been lower than it now is ; and we 

 have found in a displacement of the pole and the earth's 

 axis the only probable explanation of these phenomena. 



This cause does not, however, suffice to explain all 

 anomalies, and we must assume for all parts of the earth 

 the prevalence of a somewhat warmer climate, an in- 

 crease perhaps of a few degrees only, which manifests 

 itself particularly in the vegetation of the polar regions. 



In the Pleistocene epoch, which is, comparatively speak- 

 ing, so near to our own, the problem is so far simplified, 

 that one of the two principal factors which determined 

 the deviation from our present climatic conditions — the dis- 

 placement of the earth's axis — was no longer present ; or 

 rather, having regard to the shortness of the time that 

 has since elapsed, was so unimportant that its influence is 

 not traceable. Apart from purely local circumstances, 

 we have, as far as we can judge, only to deal with 

 uniform oscillations of temperature over the whole earth 

 anomalies of the same general character as brought 

 about the general elevation of climatic temperature in the 

 Tertiary age. 



If we follow the march of these vicissitudes ot 

 temperature, evidently determined by some cosmical 

 agency, we find at the beginning of Tertiary times a 

 moderately warm climate ; then a rise during the Eocene, 

 and then a gradual cooling, interrupted possibly by some 

 oscillations, down to a degree nearly corresponding to 

 that now prevailing, at the beginning of the Pleistocene 

 epoch. Then the cooling continued below the present 

 temperature, to a minimum at the time of the greatest 

 glaciation of the land ; then a re-warming in the inter- 

 glacial period nearly up to the present temperature ; 

 after which cold and glaciation regained the upper hand, 

 finally to give way to the present conditions, which are 

 about midway between the greatest warmth of the Ter- 

 tiary age and the greatest cold of the Pleistocene. 



One fact stands out conspicuously, viz. that these 

 changes progressed very irregularly, and were subject to 

 much oscillation, and the period during which we can 

 approximately follow the course of the change is much 

 too short to enable us to learn the law that regulated it. 

 We cannot decide whether oscillations like those of the 

 Pleistocene will be repeated, and we are now progressing 

 towards another temporary Glacial period, or whether we 

 have to expect the return of a warmer temperature such 

 as prevailed in Tertiary times, or, finally, whether the 

 outcome of all the deviations will be a lasting refrigera- 

 tion of our climate. 



Just as little can we determine at present by what 

 agency all these vicissitudes are brought about ; most 



