On the Origin of Species, by Charles Darwin, Part 4
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Insects Determine the Existence of Cattle
In Paraguay, cattle swarm southward and northward in a feral state. This is caused by the greater number of flies which lay their eggs in these animals' navels when first born. If certain insectiferous birds were to decrease in Paraguay, the parasitic insects would probably increase. In cattle and horses would become feral, and this would certainly greatly alter,. as indeed I have observed in parts of South America, the vegetation. It shows how plants and animals are bound together by a web of complex relations.
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