Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
In spite of
the differences in language and manners which naturally
were to be found within this wide territory, a close mutual intercourse,
an innate sense of fellowship, seems to have knit together
the tribes from the Rhone and Garonne to the Rhine and the Thames;
whereas, although these doubtless were in a certain measure locally
connected with the Celts in Spain and in the modern Austria,
the mighty mountain barriers of the Pyrenees and the Alps
on the one hand, and the encroachments of the Romans and the Germans
which also operated here on the other, interrupted the intercourse
and the intrinsic connection of the cognate peoples far otherwise
than the narrow arm of the sea interrupted the relations
of the continental and the British Celts.
Unhappily we are not
permitted to trace stage by stage the history of the internal development
of this remarkable people in these its chief seats; we must be content
with presenting at least some outline of its historical culture
and political condition, as it here meets us in the time of Caesar.