When I picked up this entertaining book, I thought I knew the answers to these questions. Charles Darwin did a beautiful if funky series of experiments at his country house in Kent, to explain how plants and animals might have crossed the oceans. In his day, the reigning explanation was supernatural: God put them there. Darwin’s thinking was more mundane. In the mid-1850s, he filled bottles with salt water, and added seeds and plants to find out how long they’d float, and whether they could soak in brine for months and still germinate. The seeds proved to be hardy and so did Darwin. “It is quite surprising that the Radishes shd have grown,” he wrote to a friend, “for the salt water was putrid to an extent, which I cd not have thought credible had I not smelt it myself.”