been inclosed so that cattle could not enter.

But how important an element inclosure 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 inclosed, 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 uninclosed 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, with twenty-six rings of growth, had during many years tried to
raise its head above the stems of the heath, and had failed. No wonder
that as soon as the land was inclosed 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 by some means, probably by other parasitic
insects. Hence if certain insectivorous birds were to decrease in
Paraguay, the parasitic insects would probably increase; and this would
lessen the number of the navel-frequenting flies; then cattle and horses
would become feral, and this would certainly greatly alter (as indeed I
have observed in parts of South America) the vegetation; this again
would largely affect the insects; and this, as we have just seen in
Staffordshire, the insectivorous birds