THE FALL OF THE HOUSE OF USHER What is "the remarkable fact" concerning the Usher Family?

From the text:

"I had learned, too, the very remarkable fact, that the stem of the Usher race, all time-honored as it was, had put forth, at no period, any enduring branch; in other words, that the entire family lay in the direct line of descent, and had

always, with very trifling and very temporary variation, so lain."

there house went down of fire...