In A Tale of Two Cities, what fact clinches Darnay's acquittal in Book the Second, Chapter 3?

Reading this chapter carefully reveals that the case very
quickly seems to hang on the identification of Charles Darnay in the coffee-room of a
hotel, waiting for another person. However, Sydney Carton shows his genius by
recognising that a peculiar similarity between himself and Charles Darnay exists, which
leads him to give Stryver the suggestion that, in the end, leads to Darnay's acquittal.
Stryver thus asks the witness to identify the similarity between Darnay and Carton, and
the following scene is described:

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Allowing for my learned friend's appearance
being careless and slovenly if not debauched, they were sufficiently like each other to
surprise, not only the witness, but everbody present, when they were thus brought into
comparison.

The uncanny
likeness between Darnay and Carton thus allowed Stryver to "smash this witness like a
crochery vessel, and shiver his part of the ase to useless lumber." Thus it is that the
similiarity between these two central characters is shown to be the lifeline that leads
to Darnay's salvation in this chapter, which of course foreshadows how Darnay will be
saved by their likeness once more at the end of the book.