osed, and self-sown firs are now springing up in multitudes, so close
together that all cannot live. {72} 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, 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
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
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 became feral, and this would certainly greatly {73} alter (as indeed
I have observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we
have ended with them, Not that in nature the relations can ever be as
simple as this. Battle within battle must ever be recurring with varying
success; and yet in the long-run the forces are