1. That the causes which induce the extremes of climate in the temperate zone of Northern Asia and America could not have existed in Britain. Before this objection can hold good, it is necessary to show what these causes are. Prevalence of wind in certain directions, writes Mr. Geikie (p. 166). This may be one of the causes; but at the same time it is one of the effects of change of temperature. The vera causa of the extremes in both cases seems to me to be that pointed out by Sir C. Lyell in the 12th chapter of the 'Principles' (1867)—the extension of a large mass of land from the equator to the polar circle. Such a mass of land extended from the range of Atlas northwards to the snowy regions of Scandinavia during the Pleistocene age (see Map, p. 436), the Mediterranean being reduced to two land-locked seas, and the mainland of Africa being continued on the one hand into Spain, and on the other, by Malta and Sicily, into Italy, and Greece extending so far south as to embrace Candia. Such an extent of land is surely an exact parallel to the two cases which I have quoted above; and climatal extremes must necessarily have been the result of the substitution of land for such a large area that is now covered by the sea. (See Map.) Mr. Geikie admits that at that time the winters were very much colder than they are now, because the higher mountains of Europe were also covered by mers de glace. Can he deny that the above geographical change in the Mediterranean area would have also left its mark in a higher summer temperature, other causes being put out of sight, than now? The inference that the summer temperature (p. 166) would be lower than at present in Britain, because there were glacier-areas in the north and in the higher districts in Central Europe, is inconclusive, since the mere existence of a glacier tells us nothing of the summer heat of the surrounding regions. Could we predicate, for instance, the temperature of the Subhimalayas from the contemplation of the glaciers of the central ridge? or the summer heat of Lombardy or of Provence from the glaciers of the Alps? Glaciers merely imply the existence of a certain line of mean annual temperature, above which the snow accumulates, and from which they are pushed down in some cases, as in New Zealand, to within a few hundred feet of the sea. It is obvious, therefore, that any argument from the Pleistocene glaciers to the summer heat is without value. All speculations as to the prevalent wind at that remote time in different parts of Europe appear to me mere guesses and nothing else.
2. The second objection is that the Hippopotamus is not a migratory animal. Is this so? It is true that its aquatic habits forbid its migration over the vast arid plains of Southern Africa, like the Antelopes; but, on the other hand, direct evidence for or against its migratory habits in a well-watered region is at present wanting. The remains found in Nubia, and preserved in the British Museum, certainly prove that once it ranged further north on the Nile than it does now. So far the evidence is in favour of its being a migratory animal. If its present range be compared with that during the Pleistocene it is impossible to deny that it has migrated from Africa to Yorkshire, or vice versâ.
