Had the butler dashed in the front door as he said he did, there would
have been foot-tracks in the vestibule.
Remember, the Professor ‘splashed his way through the mud and rain, to
the _door_ of 27 Holden Road,’ and found the vestibule spotless.
Therefore, Wilkins was lying, and as Cannon corroborated his story, he
was also necessarily involved.
Nay, her foot speaks.
Shakespeare.