chap, hi.] 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, hack 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 be northern plants ; and a striking proof 
of this is to be found on the Peak of Teneriffe, a mountain 
12,000 feet high. In the uppermost 4,500 feet of this mountain 
above the limit of trees, Yon 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 corresponding to 
them 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 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 the Romans. 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 Denmark 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 
