"Lively writing about science and nature depends less on the offering of good answers, I think, than on the offering of good questions," said David Quammen in the original introduction to Natural Acts. For more than two decades, he has stuck to that credo. In this updated version of his first essay collection, Quammen's lively curiosity leads him from New Mexico to Romania, from the Congo to the Amazon, asking questions about mosquitoes (what are...