"The apparent fixity of species once seemed the greatest argument against evolution, just as the apparent fixity of the earth once seemed a commonsense argument against Copernicanism. Now the satisfying and reassuring sameness that once encouraged Aesop and other fable spinners to speak of The Fox, The Owl, The Wolf, The Whale, and The Crow seem more illusory than ever before. “all in flux,: said the Greek philosopher Heraclitus; “everything flows.” The forms and instincts of living things, the invisible borders among them, and the very coasts and landscapes they inhabit are all more fluid and in more flux than even Heraclitus could have imagined."