Audubon's fourth volume, the printing of which was announced in this letter, was published in November, and at the conclusion of the introduction he said: I believe the time to be fast approaching when much of the results obtained from the inspection of the exterior alone will be laid aside; when museums filled with the stuffed skins will be considered insufficient to afford a knowledge of birds; and when the student will go forth not only to observe the habits and haunts of animals, but to procure specimens of them to be carefully dissected. These prophetic remarks, which were no doubt inspired by the studies of MacGillivray, have found ample justification in the later history of ornithology; to give a single illustration, it was through the aid of anatomy only, in this instance, of the vocal organs, that modern students have been able to define the true Passerine or Oscine birds, as distinguished from all others, and as the highest representatives of the avian class. In the autumn of 1838, when Mrs. Audubon was still suffering from impaired health, the naturalist paused in his work to indulge in a short vacation, the first in many years. On September 14, he wrote to Edward Harris, who was then in Scotland:[151] _Audubon to Edward Harris_ EDINBURGH, _14 Sep'., 1838_. MY DEAR FRIEND: Not having heard anything from you in answer to my last, I suppose that you may yet be away from Glasgow, but as we ourselves are going off tomorrow to the "Highlands," with a view to be at Glasgow on Thursday next, I write to you now, with the hope of meeting you _then_. Nothing of importance has occurred here since my last, but _the book_ has considerably swollen in its progress towards completion. We all unite in best wishes to you and I remain as ever your most truly attached and sincere friend, JOHN J. AUDUBON We intend being home again on Saturday next. [Addressed] To EDWARD HARRIS Esqr. Comrie's Royal Hotel, Glasgow. With MacGillivray as guide, Audubon and his family visited Stirling, Doun and Callander, where they "marched in a body to the Falls of Bracklin, guided by a rosy-cheeked Highland lassie, stopping now and then by the way to pick up a wild flower,—a blue-bell, a 'gowan,' or a dog-rose, or to listen to the magpies and titmice." From Callander they ascended to Loch Katrine, and explored the Trossachs, "admired by many, chiefly or entirely on account of Scott's description of them"; at the tavern there, said Audubon, "with that most curious innate desire which there is in us of becoming older, for the purpose of enjoying the morrow, I went to rest, anxious to see the morn, and discover what existed beyond the crags that had bounded my view."