Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live.
When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundred yards distant from one of the old
clumps, I counted thirty-two little trees; and one of them, judging
from the rings of growth, had during twenty-six years tried to raise
its head above the stems of the heath, and had failed. No wonder that,
as soon as the land was enclosed, it became thickly clothed with
vigorously growing young firs. Yet the heath was so extremely barren
and so extensive that no one would ever have imagined that cattle
would have so closely and effectually searched it for food.
Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born.
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