22 
DARWINISM 
CHAP. 
first place to the holm-oak, which is now giving way to the 
beech. Aspen, birch, fir, oak, and beech appear to be the 
steps in the struggle for the survival of the fittest among the 
forest-trees of Denmark. 
It may be added that in the time of the Romans the 
beech Avas the principal forest-tree of Denmark as it is now, 
while in the much earlier bronze age, represented by the later 
remains found in the peat bogs, there Avere no beech-trees, or 
very feAv, the oak being the prevailing tree, while in the still 
earlier stone period the fir Avas the most abundant. The 
beech is a tree essentially of the temperate zone, having its 
northern limit considerably southward of the oak, fir, birch, 
or aspen, and its entrance into Denmark Avas no doubt due to 
the amelioration of the climate after the glacial epoch had 
entirely passed aAvay. We thus see how changes of climate, 
which are continually occurring owing either to cosmical or 
geographical causes, may initiate a struggle among plants 
which may continue for thousands of years, and which must 
profoundly modify the relations of the animal world, since 
the very existence of innumerable insects, and even of many 
birds and mammals, is dependent more or less completely on 
certain species of plants. 
The Struggle for Existence on the Pampas. 
Another illustration of the struggle for existence, in which 
both plants and animals are implicated, is afforded by the 
pampas of the southern part of South America. The absence 
of trees from these vast plains has been imputed by Mr. 
DarAvin to the supposed inability of the tropical and sub¬ 
tropical forms of South America to thrive on them, and there 
being no other source from which they could obtain a supply ; 
and that explanation Avas adopted by such eminent botanists 
as Mr. Ball and Professor Asa Gray. This explanation has 
always seemed to me unsatisfactory, because there are ample 
forests both in the temperate regions of the Andes and on the 
Avhole Avest coast down to Terra del Fuego; and it is inconsistent 
with what Ave knoAv of the rapid variation and adaptation of 
species to ncAv conditions. What seems a more satisfactory 
explanation has been given by Mr. Edwin Clark, a civil 
engineer, avIio resided nearly tAvo years in the country and 
