This must be serious, Watson. A death which has caused mybrother to alter his habits can be no ordinary one. What in theworld can he have to do with it? The case was featureless as Iremember it. The young man had apparently fallen out of thetrain and killed himself. He had not been robbed, and there wasno particular reason to suspect violence. Is that not so?

I have it, I cried, and plunged among the litter of papers upon
the sofa. Yes, yes, here he is, sure enough! Cadogen West was
the young man who was found dead on the Underground on Tuesday
morning.

They began by using him as a short-cut, aconvenience; now he has made himself an essential. In that greatbrain of his everything is pigeon-holed and can be handed out inan instant.