one quite different soil to another. Besides a great change in the
proportional numbers of the native heath-plants, twelve species which
could not be found on the heath flourished in the plantations. The
effect on the insect life must have been still greater, for six
insectivorous birds which were very common in the plantations were not
to be seen on the heath, which was, however, frequented by two or three
different species of insectivorous birds. It would have required
continued study for several years to determine all the differences in
the organic life of the two areas, but the facts stated by Mr. Darwin
are sufficient to show how great a change may be effected by the
introduction of a single kind of tree and the keeping out of cattle.

The next case I will give in Mr. Darwin's own words: "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 numbers, 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 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. 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