“There’s a case in it,” I said, “for the District Court of the United States, on the criminal side, or I’m a poor detective.”

“All detectives are poor,” said Walker. “If they were rich, they would have a town house, a country place and a string of hunters.”

“Well,” I said, “that’s what the old boy in the taxicab has got; and he’s got something else that the United States doesn’t allow him to take across a state line.”

Walker looked at me queerly. He put the tip of his finger to his forehead.

“Touch of the heat?”

“Look here,” I said, “isn’t this sort of thing just as much in your line of duty as trying to prevent the crooked cashier from boring from within? Isn’t the United States by a fairly recent statute, helping virtue to evade the dragon?”

Walker’s face wrinkled into a twisted smile.

“It’s helping the clever fille de joie to levy a little blackmail on the side.”

“Wrong dope, in this instance,” I said.

I began to describe to him the incident and the two persons. I described them carefully, minutely, and he listened without a word and without a motion. He stood perfectly still, there in the hot street before Bartoldi’s mammoth shop.