grasses and carices) flourished in the plantations, which could
not be found on the heath. The effect on the insects must have been
still greater, for six insectivorous birds were very common in the
plantations, which were not to be seen on the heath; and the heath was
frequented by two or three distinct insectivorous birds. Here we see how
potent has been the effect of the introduction of a single tree, nothing
whatever else having been done, with the exception that the land had
been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. 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. The increase of these flies, numerous as they are,
must be habitually checked