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If agricultural land be left uncultivated, in a few years the jungle returns, and signs are not lacking that a similar danger is always lying in wait for the fields of thought, which, by the labour of three hundred years, have been cleared and brought into cultivation by men of science. The destruction of a very small percentage of the population would suffice to annihilate scientific knowledge, and lead us back to almost universal belief in magic, witchcraft and astrology.
