CHAP. III.] CONDITIONS AFFECTING DISTRIBUTION. 43 



As the icy mantle gradually melted off the face of the earth 

 these plants would occupy the newly exposed soil, and would 

 thus necessarily travel in two directions, back towards the arctic 

 circle and up towards the alpine peaks. The facts are thus 

 exactly explained by a cause which independent evidence has 

 proved to be a real one, and every such explanation is an addi- 

 tional proof of the reality of the cause. But this explanation im- 

 plies, that in cases where the Glacial epoch cannot have so acted 

 alpine plants should not he northern plants ; and a striking proof 

 of this is to be found on the Peak of Tenerilfe, a mountain 

 12,000 feet higli. In the uppermost 4,500 feet of this mountain 

 above the limit of trees, Von Buch found only eleven species of 

 plants, eight of which were peculiar ; but the whole were allied 

 to those found at lower elevations. On the Alps or Pyrenees at 

 this elevation, there would be a rich flora comprising hundreds 

 of arctic plants ; and the absence of anything correspondiug to 

 them in this case, in which their ingress was cut oft' by the sea, 

 is exactly what the theory leads us to expect. 



Changes of Vegetation as affecting the Distribution of Animeds. 

 — As so many animals are dependent on vegetation, its changes 

 immediately affect their distribution. A remarkable example of 

 this is afforded by the pre-historic condition of Denmark, as 

 interpreted by means of the peat-bogs and kitchen-middens. 

 This country is now celebrated for its beech-trees; oaks and pines 

 being scarce ; and it is known to have had the same vegetation in 

 the time of tlie Eomans. In the peat-bogs, however, are found 

 deposits of oak trees ; and deeper still pines alone occur. Now 

 the kitchen-middens tell us much of the natural history of 

 Denmark in the early Stone period ; and a curious confirmation 

 of the fact that Demnark like Norway was then chiefly covered 

 with pine forests is obtained by the discovery, that the Caper- 

 cailzie was then abundant, a bird which feeds almost exclusively 

 on the young shoots and seeds of pines and allied plants. The 

 cause of this change in the vegetation is unknown ; but from the 

 known fact that when forests are destroyed trees, of a different 

 kind usually occupy the ground, we may suppose that some such 

 change as a temporary submergence might cause an entirely 



