If the climate, since the Glacial period, has ever been in any degree warmer than at present (as some geologists in the United States believe to have been the case, chiefly from the distribution of the fossil Gnathodon), then the arctic and temperate productions will at a very late period have marched a little further north, and subsequently have retreated to their present homes; but I have met with no satisfactory evidence with respect to this intercalated slightly warmer period, since the Glacial period.
The arctic forms, during their long southern migration and re-migration northward, will have been exposed to nearly the same climate, and, as is especially to be noticed, they will have kept in a body together; consequently their mutual relations will not have been much disturbed, and, in accordance with the principles inculcated in this volume, they will not have been liable to much modification.
In illustrating what, as I believe, actually took place during the Glacial period, I assumed that at its commencement the arctic productions were as uniform round the polar regions as they are at the present day.
Consequently we have here everything favourable for much modification,--for far more modification than with the Alpine productions, left isolated, within a much more recent period, on the several mountain-ranges and on the arctic lands of the two Worlds.
Watson has recently remarked, 'In receding from polar towards equatorial latitudes, the Alpine or mountain floras really become less and less arctic.
But I do not doubt that some temperate productions entered and crossed even the lowlands of the tropics at the period when the cold was most intense,--when arctic forms had migrated some twenty-five degrees of latitude from their native country and covered the land at the foot of the Pyrenees.
Although we have reason to believe from geological evidence that the whole body of arctic shells underwent scarcely any modification during their long southern migration and re-migration northward, the case may have been wholly different with those intruding forms which settled themselves on the intertropical mountains, and in the southern hemisphere.
As the tide leaves its drift in horizontal lines, though rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountain-summits, in a line gently rising from the arctic lowlands to a great height under the equator.
This ocontent-rich snapshoto of the developing Arctic
is an updated and expanded version of the report Strategic Assessment of Development of the Arctic
: An assessment conducted for the European Union (SADA), which is the final report of an assessment conduced within the oStrategic Environmental Impact Assessment of Development of the Arctico project financed by the European Commission Directorate-General for the Environment, and carried out by a network of 19 leading Arctic
universities, research and communication centers with extensive activities in and knowledge of the Arctic