Beyond the description which makes him resemble Monsieur Darzac, we know nothing.
"I have announced in the leading journals that a handsome reward will be given to a driver of any public conveyance who drove a fare to No.

40, Post Office, about ten o'clock on the morning of the 24th of October.
Information to be addressed to 'M.

R.,' at the office of the 'Epoque'; but no answer has resulted.

The man may have walked; but, as he was most likely in a hurry, there was a chance that he might have gone in a cab.
Who, I keep asking myself night and day, is the man who so strongly resembles Monsieur Robert Darzac, and who is also known to have bought the cane which has fallen into Larsan's hands?
"The most serious fact is that Monsieur Darzac was, at the very same time that his double presented himself at the Post Office, scheduled for a lecture at the Sorbonne.

He had not delivered that lecture, and one of his friends took his place.