Inheritance of the title "Marquis St. Evrémonde" in "A Tale of Two Cities"

Question

In A Tale of Two Cities, the title "Marquis St. Evrémonde" is held by Darnay's uncle, but had previously been held by Darnay's father, for according to Dr. Manette (book III, chapter X) Darnay's mother had been "the wife of the Marquis":

“The lady was young, engaging, and handsome, but not marked for long life. She was in great agitation. She presented herself to me as the wife of the Marquis St. Evrémonde. [...] When I handed her down to the door, there was a child, a pretty boy from two to three years old, in her carriage. [...] She kissed the boy, and said, caressing him, ‘It is for thine own dear sake. Thou wilt be faithful, little Charles?’

How does the inheritance of the title work? If basic firstborn male inheritance was in play, Darnay would be the Marquis at the time of the main story, but the title seems to have gone laterally within his father's generation rather than being passed down to him.

(Alternatively, I would be happy with an answer that pointed at some critical commentary treating Darnay’s mother’s reference to her being the wife of the Marquis as an error that should have been something like “wife of the Marquis’ brother”.)

Answer 1

Darnay says to his uncle (book II, chapter IX):

“Why need I speak of my father’s time, when it is equally yours? Can I separate my father’s twin-brother, joint inheritor, and next successor, from himself?”

So Darnay’s father and uncle were twin brothers and “joint inheritors”. Joint inheritance was a system practiced by various groups in the medieval period, notably the Franks (under the original Salic law) and the Lombards, whereby all the sons became co-inheritors of the hereditary estate. So Dickens requires the reader to imagine that the Marquisate of Saint Evrémonde has, by some fluke of history, descended from such a group to the eighteenth century, without ever becoming subject to primogeniture.

This is a bit of an implausible pill to swallow, but it’s far from the most unlikely of the melodramatic devices in the novel! The law of land tenure and inheritance does tend to be a patchwork of local rules, for example, even in Dickens’ time the system of gavelkind applied in Kent, but not in the rest of England.