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Darwin had observed so much variety in creatures traditionally assumed to be one species that his sense of a hard line between species had slowly begun to dissolve. Even that most sacred line, the supposed inability of different species to create fertile offspring, he realized was bunk. “It cannot be maintained that species when intercrossed are invariably sterile,” Darwin writes, “or that sterility is a special endowment and sign of creation.” Leading him finally to declare that species—and indeed all those fussy ranks taxonomists believe to be immutable in nature (genus, family, order, class, etc.)—were human inventions. Useful but arbitrary lines we draw around an ever-evolving flow of life for our “convenience.” “Natura non facet salute,” he writes. Nature doesn’t jump. Nature has no edges, no hard lines.
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Lulu Miller (Why Fish Don’t Exist: A Story of Loss, Love, and the Hidden Order of Life)