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 was 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 few, the oak being the prevailing tree, while in the still 

 earlier stone period the fir was 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 was no doubt due to 

 the amelioration of the climate after the glacial epoch had 

 entirely passed away. 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. 

 DarAWn 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 ex2Dlanation has 

 always seemed to me unsatisfactory, because there are ample 

 forests both in the temperate regions of the Andes and on the 

 whole west coast down to Terra del Fuego; and it is inconsistent 

 with what we know of the rapid variation and adaptation of 

 species to new conditions. What seems a more satisfactory 

 explanation has been given by Mr. Ed^vvin Clark, a civil 

 engineer, Avho resided nearly two years in the country and 



