"Why, all our art treasures of today are only dug up common places of three or four hundered years ago"? How does the writer prove this with reference to the China Dog?

Jerome is referring to items we call antique and art are merely junk that people have thrown away in the past. He refers to a silly "China Dog" (ceramic ornament) in his bedroom that people make fun of. Jerome speculates that in 200 years, someone will dig it up and call it art.