What a fascinating creature an old marriage is! Sally thought. From its little, squirming beginning all those years ago at the Ship Ahoy it had changed over time into this highly evolved being. Buffeted by the demands of married life, useful traits had been selected for, modified, and strengthened. Dave’s sultry sluggishness that had been so attractive in courtship had been honed by need into an admirable, stoic patience. And what had been in her youth a nearsighted preoccupation with detail had turned into a useful efficiency that they both counted on. Other traits, of diminishing usefulness, such as that Elvis Presley falling in love, were almost gone, hardly noticeable now—but still there in a way, like the little vestigial leg bones embedded in the body wall of a whale.

And here their marriage was today, after all those adaptive shifts—a funny-looking, old thing, surely, with splayed toes and eyes on the top of its head, but elegantly suited to its ecological niche.

“What could Dave be thinking?” Sally said to herself. A divorce in such a marriage would be like the extinction of a species.

Late that night Sally heard a thump, an anticipatory pause, then another thump and a pause, then three in a row—thumpthumpthump. From the far side of the house she could barely hear the response: thump, thump, thumpthumpthump. The rats had moved in.

The next day Ethel baited and set out the traps. “These rats are nobody’s fool,” she said, and sure enough, that afternoon it turned gray and dismal, a mean rain fell until dark, and the temperature dropped forty degrees.

“This is just like the first freeze of 1950,” said Ethel. “This will be a good year for trapping rats.”

Just before midnight Sally heard the rustling sound, a tentative testing—a soft nudge on metal, then the flap of the trapdoor, and the panicked scrambling. The lights came on, and there was Ethel, wild-eyed and triumphant in her chaotic floral bathrobe with her white hair shoved up both sides of her head.

“Look at that!” she said, tilting the trap so the rat ran up the side. “A big bull rat. Look at that coat, look at the gleam in his eye—the size of him!” She looked at the rat and the rat looked at her. “I don’t think I’ve ever caught a finer-looking rat.

“We have to turn him loose way out from civilization. A rat like this can travel ten miles in a day and destroy an entire household in one night. You drive.”