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Doctrine of signatures: (above) a plant (arum) with seed-pods resembling the horn of a unicorn, and (below) a unicorn’s head. Coloured ink drawing, c. 1923, after G.B. Della Porta. Credit: Wellcome Collection. CC BY

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Doctrine of signatures: (above) a fruit (pomegranate) resembling the human jaw, and (below) a jaw-bone and teeth. Coloured ink drawing, c. 1923, after G.B. Della Porta. Credit: Wellcome Collection. CC BY

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Doctrine of signatures: (above) a plant with roots resembling the human hand, and (below) a hand. Coloured ink drawing, c. 1923, after G.B. Della Porta. Credit: Wellcome Collection. CC BY

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Doctrine of signatures: (above) a plant with roots resembling a crayfish, and (below) a crayfish. Coloured ink drawing, c. 1923, after G.B. Della Porta. Credit: Wellcome Collection. CC BY

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Doctrine of signatures: (above) a plant (clover) with leaves resembling the horns of a goat, and (below) a goat’s head. Coloured ink drawing, c. 1923, after G.B. Della Porta. Credit: Wellcome Collection. CC BY