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 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, and so onward in ever-increasing
circles of complexity. We began this series by insectivorous birds, and we
had 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 so nicely balanced that the face of
Nature remains uniform for long periods of time, though assuredly the
merest trifle would often give the victory to one organic being over
another. Nevertheless, so profound is our ignorance, and so high our
presumption, that we marvel when we hear of the extinction of an organic
being; and as we do not see the cause, we invoke cataclysms to desolate the
world, or invent laws on the duration of the forms of life!"--(pp. 72, 73.)

"When we look at the plants and bushes clothing an entangled bank, we arc
tempted to attribute their proportional numbers and kinds to what we call
chance. But how false a view is this! Every one has heard that when an
American forest is cut down, a very different vegetation springs up; but it
has been observed that the trees now growing on the ancient Indian mounds,
in the Southern United States, display the same beautiful diversity and
proportion of kinds as in the surrounding virgin forests. What a struggle
between the several kinds of trees must here have gone on during long
centuries, each annually scattering its seeds by the thousand; what war