The geographical distribution of animals, with a study of the relations ...

chap, ill.] CONDITIONS AFFECTING DISTRIBUTION. 43As the icy mantle gradually melted off the face of the earththese plants would occupy the newly exposed soil, and wouldthus necessarily travel in two directions, back towards the arcticcircle and up towards the alpine peaks. The facts are thusexactly explained by a cause which independent evidence hasproved to be a real one, and every such explanation is an additionalproof of the reality of the cause. But this explanation implies,that in cases where the Glacial epoch cannot have so actedalpine plants should not be northern plants ;and a striking proofof this is to be found on the Peak of Teneriffe, a mountain12,000 feet high. In the uppermost 4,500 feet of this mountainabove the limit of trees, Von Buch found only eleven species ofplants, eight of which were peculiar ; but the whole were alliedto those found at lower elevations. On the Alps or Pyrenees atthis elevation, there would be a rich flora comprising hundredsof arctic plants ; and the absence of anything corresponding tothem in this case, in which their ingress was cut off by the sea,is exactly what the theory leads us to expect.Changes of Vegetation as affecting the Distribution of Animals.—As so many animals are dependent on vegetation, its changesimmediately affect their distribution. A remarkable example ofthis is afforded by the pre-historic condition of Denmark, asinterpreted by means of the peat-bogs and kitchen-middens.This country is now celebrated for itsbeech-trees; oaks and pinesbeing scarce ; and it is known to have had the same vegetation inthe time of the Eomans. In the peat-bogs, however, are founddeposits of oak trees ; and deeper still pines alone occur. Nowthe kitchen-middens tell us much of the natural history ofDenmark in the early Stone period ; and a curious confirmationof the fact that Denmark like Norway was then chiefly coveredwith pine forests is obtained by the discovery, that the Capercailziewas then abundant, a bird which feeds almost exclusivelyon the young shoots and seeds of pines and allied plants. Thecause of this change in the vegetation is unknown ; but from theknown fact that when forests are destroyed trees, of a differentkind usually occupy the ground, we may suppose that some suchchange as a temporary submergence might cause an entirely