equented 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 multitudes 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 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 birds. Hence
if certain insectivorous birds (whose numbers are probably regulated by
hawks or beasts of prey) were to increase in Paraguay, the flies would
decrease,--then cattle and horses would become feral; and this would
certainly greatly alter (as ind