Page:Journal of botany, British and foreign, Volume 9 (1871).djvu/368

 340 GLACIAL ORIGIN OF THE PEAT-BOGS OF THE JUllA.

boreal character and the nature of the soil that supports it, shows clearly that it is Pliocene and contemporary with the olacial era. The opinion of the geologists and botanists who suppose that at this era the vegetation of the regions of the globe which were invaded by the ice was absolutely null, does not appear to me well founded. In fact, the glacial epoch exists still at the two poles. Eound the Arctic pole, Greenland, Spitzbergen, and Nova Zembla, are covered with masses of ice, of which the branches stretch down to the sea, but still they have plants growing in the intervals which the ice does not cover. In Spitzbergen there are 93 Phanerogamia ; in Greenland, 328. Eobert Brown, who visited the Bay of Disco, in 70° N. lat., collected 129 species there in the course of a single summer. These plants are not all special to the Arctic region, but many spread out towards the south; and, if we glance over the catalogue of these plants of the Jurassic turf-bogs, we find out of a total of 179 species, 73 are actually Arctic at the present day; besides these, there are 1 06 species not now found in the Arctic regions, but all of them, except Swe/iio, are Scandinavian. Now, when we remember that the Neufchatelois Jura is 23° of latitude south of Lapland, is it absurd to suppose that at the glacial epoch its cli- mate was not more rigorous than that of Lapland is now, and that these Scandinavian species, of which the centre of creation is still to deter- mine, existed also at the epoch when the alpine glaciers overtopped the Jurassic ridges ? I have elsewhere made the calculation that if the mean temperature of Geneva were lowered four degrees Centigrade, the glaciers of the Alps would again invade the basin of the Leman. If this were the case, the temperature of the Jurassic valleys would still be higher than that of Altenfiord, in 70° N. lat., where most of the species grow which Ave find now in the Jurassic peat-bogs. The plants have persisted in the Jura in spite of a warming of the climate, amounting to four degrees, be- cause they have found in the constitution and humidity of the soil, condi- tions of existence analogous to those with which they are surrounded in Lapland at the present day.

Another proof that the climate of the glacial epoch, during which the erratic blocks of the Jura were laid down, was not rigorous enough to exclude all vegetation, is found in the fact that man inhabited the basin of the Leman, and that of the lakes of Zurich and Constance, immediately after the glacial epoch at Vierier, Meilen, and Schussenried, where we find worked Hints and reindeer bones in the alluvium of the terraces just above the glacial beds. But if, as I have always believed, the old alluvium of Switzerland is contemporaneous with the same deposit in the plains of France, w^here incontestable proofs of the existence of man have been ob- tained, I do not despair of hearing that the Swiss geologists have found human bones and worked flints either in the midst of the glacial beds or even in the old alluvium on which they rest.

But to return to our plants. By the area of their geographical distri- bution, they furnisii us with an argument which is not without value. Spread through northern Europe, many of them stop at the Pyrenees and Alps. In North America they stop generally at the fortieth degree of latitude. Now these are the extreme limits of the great extension of glaciers of which the Arctic pole and the great European mountain-chains, situated at the north of 42°, were the principal centres. The plants, therefin-e, grow now in the very regions then invaded by the glaciers.

We may say, perhaps, that the vegetation of the Jurassic turf-bogs, is

�� �