Page:Natural History Review (1862).djvu/273

258 While, however, admitting these two possible objections, it is still, I think, felt by most Palæontologists, that though the presence of one Arctic species would scarcely perhaps justify any very decided inference as to climate, still that the co-existence of such a group as this; the musk ox, the reindeer, the lemming, the Myodes torquatus, the Siberian mammoth, and its faithful companion the woolly haired rhinoceros, decidedly indicates, even though it may not prove, the existence of a climate unlike that now prevailing in Western Europe. But when, in addition, we get the physical evidence brought forward by Mr. Prestwich, the disturbed condition of the beds, and the presence of the large blocks, the inference is much strengthened. The amount of difference still remains to be ascertained. Taking the present range of the Musk ox and Reindeer as his guides, Mr. Prestwich assumes a difference in the mean winter temperature of 19° to 29°. While, however, admitting the probability of a somewhat greater winter cold, we are not, I think, yet in a position to estimate the amount of change.

It must always be borne in mind that the temperature of Western Europe is at present exceptionally mild; if we go either to the east or west, to Canada or Siberia, we find countries under the same latitude as London and Paris suffering under a far more severe climate.

The river St. Lawrence, to which I have pointed as throwing so much light on the transport of the blocks now in question, is actually in a lower latitude than the Seine or the Somme. Moreover, geologists are agreed that at the period of the boulder day, a period immediately preceding that now under consideration, the cold in Western Europe must have been far more intense than it is at present. The subject is treated at length in an excellent paper by Mr. Hopkins (then President of the Geological Society), and it is admitted (p. 61) that many of our rivers have probably followed their present directions "ever since the glacial period." Mr. Prestwich's hypothesis involves therefore in reality no change of climate. He only supposes that, in this early period of our rivers, the temperature of Western Europe agreed with that which had preceded, rather than with that which now prevails; or rather, perhaps, that, in this intermediate period, the temperature had neither the extreme severity of the glacial era, nor the exceptional mildness of modern times.

But though diminishing the improbability of the suggestion, these considerations throw no light on the alteration of the conditions which must have taken place to produce an alteration of climate so great as that inferred by Mr. Prestwich.

The principal causes which have been suggested are the following:—