Partula snails all but vanished from Polynesia after the arrival of a carnivorous foreign snail. But a global alliance of zoos has worked to bring them back. In Patricia Highsmith’s unnerving 1964 short story “The Snail-Watcher,” a broker’s quiet fascination with breeding garden snails curdles into a terrifying biological reckoning that ends with him entombed by his own neglected and ever-multiplying subj... [3087 chars]