“It’s a mystery. It got me interested in the phenomenon, though, and I started looking into it. The technical term is hyperdactyly, and there have been lots of famous people who’ve had it. It’s unclear whether it’s true or not, but there was some evidence that Hideyoshi Toyotomi, the famous leader of the Sengoku period, had two thumbs. There are plenty of other examples. There was a famous pianist who had the condition, a novelist, an artist, a baseball player. In fiction, Hannibal Lecter of The Silence of the Lambs had six fingers. It’s not all that unusual, and genetically it’s a dominant trait. There are variations among different races, but in general, one out of every five hundred people is born with six fingers. As I said, though, the vast majority of their parents have the extra fingers amputated before their children’s first birthdays, when kids begin to develop fine-motor skills. So we hardly ever run across someone with the condition. It was the same for me. Until that jar was found in the station, I’d never even heard of such a thing.”

“It is strange, though,” Tsukuru said. “If having six fingers is a dominant trait, then why don’t we see more people with them?”

The stationmaster inclined his head. “Don’t know. That kind of complicated stuff is beyond me.”

Sakamoto, who’d eaten lunch with them, opened his mouth for the first time. Hesitantly, as if rolling away a massive stone that blocked the mouth of a cave. “I wonder if you wouldn’t mind if I venture an opinion?”

“Of course,” Tsukuru said, taken by surprise. Sakamoto was not the type of young man who voiced his own opinion in front of others. “Go right ahead.”

“People tend to misunderstand the meaning of the word ‘dominant,’ ” Sakamoto said. “Even if a certain tendency is dominant, that doesn’t mean it becomes widespread throughout the population. There are quite a few rare disorders where genetically there is a dominant gene, but these conditions don’t, as a result, become common. Thankfully, in most cases these are checked at a fixed number, and remain rare disorders. Dominant genes are nothing more than one among many elements in tendency distribution. Other elements would include the survival of the fittest, natural selection, and so on. This is personal conjecture, but I think six fingers are too many for human beings. For what the hand has to do, five fingers are all that are necessary, and the most efficient number. So even if having six fingers is a dominant gene, in the real world it only manifests in a tiny minority. In other words, the law of selection trumps the dominant gene.”

After holding forth at such length, Sakamoto stepped back into silence.