It's a sentence from Bartleby, the Scrivener.
He lives, then, on ginger-nuts...he must be a vegetarian then; but no; he never eats even vegetables, he eats nothing but ginger-nuts.
What I cannot understand is why the comma has been used between two clauses "he never eats even vegetables, he eats nothing but ginger-nuts." I thought there has to be period instead of comma so that the sentence will look like this:
He lives...he never eats even vegetables. He eats nothing but ginger-nuts.
Is there a specific grammar rule that allows a comma between two clauses that are not connected by coordinate conjunctions? If there is any, please let me know.