Kennedy was still busy when I rejoined him, a little late
purposely, since I knew that he would be over his head in work.

"What's this--a zoo?" I asked, looking about me as I entered the
sanctum that evening.

There were dogs and guinea pigs, rats and mice, a menagerie that
would have delighted a small boy. It did not look like the same
old laboratory for the investigation of criminal science, though I
saw on a second glance that it was the same, that there was the
usual hurly-burly of microscopes, test-tubes, and all the
paraphernalia that were so mystifying at first but in the end
under his skilful hand made the most complicated cases seem
stupidly simple.

Craig smiled at my surprise. "I'm making a little study of
intestinal poisons," he commented, "poisons produced by microbes
which we keep under more or less control in healthy life. In death
they are the little fellows that extend all over the body and
putrefy it. We nourish within ourselves microbes which secrete
very virulent poisons, and when those poisons are too much for us-
-well, we grow old. At least that is the theory of Metchnikoff,
who says that old age is an infectious chronic, disease. Somehow,"
he added thoughtfully, "that beautiful white kitchen in the Pitts
home had really become a factory for intestinal poisons."

There was an air of suppressed excitement in his manner which told
me that Kennedy was on the trail of something unusual.

"Mouth murder," he cried at length, "that was what was being done