esented by the later remains found in the peat bogs,
there were 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. Darwin 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 was 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 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. Edwin Clark, a civil
engineer, who resided nearly two years in the country and paid much
attention to its natural history. He says: "The peculiar characteristics
of these vast level plains which descend from the Andes to the great
river basin in unbroken monotony, are the absence of rivers or
water-storage, and the periodical occurrence of droughts, or 'siccos,'
in the summer months. These conditions determine the singular character
both of its flora and fauna.

"The soil is naturally fertile and favourable for the growth of trees,
and they grow luxuriantly wherever they are protected. The eucalyptus is
covering large tracts wherever it is enclosed, a