"It is not improbable..."
The handwriting is T.O. Mabbott's.
Papers of Thomas Ollive Mabbott, Special Collections, University of Iowa Libraries; "The Murders in the Rue Morgue," in Collected Works of Edgar Allan Poe, Vol. II: Tales and Sketches, ed. Thomas Ollive Mabbott (Cambridge: The Belknap Press of Harvard University Press, 1978), 521-574.