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 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 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!
I am tempted to give one more instance showing how plants and animals,
most remote in the scale of nature, are bound together by a web of
complex relations.
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