Tristram starts out by quoting a Dr. Joseph Hall who insisted that man should not praise himself. Good advice, right? No one likes a braggart.
But what if you've done something really awesome? The talented Mr. Tristram finds himself in a tricky situation.
His writing is so skillful that not only has he written a masterful digression, but he's managed to keep the story progressing at the same time. Dinah might have seemed to take the narrative off course, but Tristram was able to portray Toby at the same time.
Digressions are the best part of reading, but they're hard on writers. Tristram is so skillful that he's managed to mix digression with progression so deftly that the novel has become a perpetual motion machine.