earched 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, and so onwards in
ever-increasing circles of complexity. Not that under nature the
relations will ever be as simple as this. Battle within battle must be
continually recurring with varying success; and yet in the long-run the
forces are so nicely balanced that the face of nature remains for long
periods of time uniform, though assuredly the merest trifle would 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!

I am tempted to give one more instance showing how plants and animals,
remote in the scale of nature, are bound together by a web of complex
relations. I shall hereafter have occasion to show that the exotic
Lobelia fulgens is never visited in my garden by insects, and
consequently, from its peculiar structure, never sets a seed. Nearly
all our orchidaceous plants absolutely require the visits of insects
to remove their pollen-masses and thus to fertilise them. I find
from experiments that humble-bees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do
not visit this flower. I have also found that the visits