comma after introductory phrase: have rules changed since 19th C.?

Is the way we put commas after introductory clauses in modern English fairly new to the language? The reason I ask is because I've been reading the essays of Mark Twain lately, and he rarely ever used them. For example, here's one from In Defense of Harriet Shelley:

"On the credit side of the account we have strong opinions from the people who knew her best."

It would (at least now, anyway) be common practice to put a comma after "account," but is that due to the changing of punctuation rules in the English language since the 19th century?

Thanks.

I think putting a comma after 'account' is unnecessary, but maybe I'm old-fashioned.

Oh, so it is a matter of the comma there being more of a modern thing, then?