Paul put the mouse on John’s hand.

“What have you been feeding her?” John asked.

“Same as the other mice. Look at this.” Paul showed him the charts he’d graphed, like Mr. Finley, a gentle upward ellipse between the X and Y axis—the slow upward climb in body weight from one generation to the next.

“One of my F2s tipped the scales at forty-five grams, so I bred him to the biggest females, and they made more than fifty babies. I weighed them all at a hundred days and picked the biggest four. I bred them and did the same thing the next generation, choosing the heaviest hundred-day weights. I got the same bell-curve distribution—only the bell was shifted slightly to the right. Bertha was the biggest of them all.”

John looked at Paul in horror. “That works?”

“Of course it works. It’s the same thing people have been doing with domestic livestock for the last five thousand years.”

“But this didn’t take you thousands of years.”

“No. Uh, it kind of surprised me it worked so well. This isn’t even subtle. I mean, look at her, and she’s only an F4. Imagine what an F10 might look like.”

“That sounds like evolutionism.”

“Don’t be silly. It’s just directional selection. With a diverse enough population, it’s amazing what a little push can do. I mean, when you think about it, I hacked off the bottom 95 percent of the bell curve for five generations in a row. Of course the mice got bigger. I probably could have gone the other way if I’d wanted, made them smaller. There’s one thing that surprised me, though, something I only noticed recently.”

“What?”

“When I started, at least half of the mice were albino. Now it’s down to about one in ten.”

“Okay.”

“I never consciously decided to select against that.”

“So?”

“So, when I did culls … when I decided which ones to breed, sometimes the weights were about the same, and I’d just pick. I think I just happened to pick one kind more than the other.”