BIOLOGY has returned to the nation’s highest court. It’s not Darwin’s theory of evolution on the docket this time, but the nature of sex. Defenders of Proposition 8, California’s ban on gay marriage, base their case on what they call the “objective biological fact” that procreation is an exclusively heterosexual process. Citing the 18th-century English jurist William Blackstone, they argue that marriage should be “founded in nature.”

This invocation of nature echoes other voices. Last December, before Pope Benedict XVI resigned, he used his Christmas greetings to the Roman Curia to deplore what he called a “new philosophy of sexuality” that manipulates and denies nature. Roy S. Moore, re-elected last fall as the Chief Justice of the Alabama Supreme Court, once let rip with less measured language, exclaiming in a child-custody case that homosexuality was “a crime against nature and a violation of the laws of nature and of nature’s God.” Meanwhile, Tennessee legislators have repeatedly sought the prohibition of any sexual education “inconsistent with natural human reproduction.” None of this is, in fact, new: Oscar Wilde’s trials hinged on the courts’ understanding of natural love and unnatural vice.

References to biology coat these arguments with a gloss of scientific rigor. But before we write nature into law, let’s take a stroll outside the Supreme Court’s chambers and check those biological facts. Descending the steps of the court, we enter Washington’s planted landscape, a formal park where nature stands alongside patriotic monuments and federal buildings. There is no shortage of counsel about biology here.

The grandeur of the National Mall is rightly famous. Less well known are the hermaphroditic sex lives of many of its inhabitants. Japanese cherry trees break bud in explosions of pink; male and female coexist at the heart of each flower. The American elms that frame the Mall’s lawns present a more reserved countenance to the world. But their inconspicuous lime-green flowers are biologically bisexual. Ginkgo, another tree common in Washington, follows a Prop 8-approved sexual separation, growing as discrete males and females. But even the ginkgo will sometimes surprise horticulturalists with a stray flower of the other sex.