When William MacGillivray first met Audubon, in the autumn of 1830, he was an enthusiastic naturalist of four and thirty, young, but, as we have seen, a thorough anatomist, who stood firmly on his own feet and was destined to advance his favorite study in a notable degree. Audubon at this time was forty-five, but in anatomy the older man gladly sat at the feet of the younger and acknowledged him master; while this young anatomist was dissecting, Audubon in the _rôle_ of student was seated by his side, and we may be sure that little escaped his penetrating eye and keen intelligence. To MacGillivray, on the other hand, Audubon was master of his art, and to him he looked for criticism of his own artistic efforts; after him he named a son, and to him dedicated a child of his brain.[125] In short, MacGillivray looked upon Audubon as his best friend in the world, and the latter fully appreciated his indebtedness to this able assistant. MacGillivray continued to aid Audubon with his letterpress, revising and probably contributing most of the technical details; in the fourth and concluding volumes, published in 1838 and 1839, the large store of anatomical matter and many excellent drawings were duly acknowledged as coming from his hand. His own writings were varied and numerous, but were generally characterized by a high degree of excellence. His _History of British Birds_, in five volumes (1837-1852), was too extended and too technical ever to become popular, but in that work, for the first time in the history of science, classification was placed on a strictly anatomical basis. MacGillivray even followed Audubon to some extent by introducing into this work "delineations of British scenery and character," but under another head. The sixth of his "Lessons in Practical Ornithology" recounts in dialogue form the experiences of two friends in tramping the Pentland Hills together; says "Physiophilus" [himself], "You must have many fine songsters in America"; to which "Ornithologus" [Audubon] replies: That we have indeed. The Mocking Bird, of course, stands first in my opinion, and is unrivalled. Then, perhaps on account of my own sensitive nature, I would place next the Wood Thrush, although the Cat Bird is far its superior in many points, as is also the Turdus rufus. Think of our Rose-breasted Pine and Blue Grosbeaks, how mellow and sweet their continuous songs are, whether by day or during calm nights. Watch the varied ditties of the Orchard Oriole, and the loud and more musical notes of its brother, the Golden Hangnest. You have never heard the Tawny Thrush or the Hermit Thrush, otherwise, believe me, you would have enjoyed much delicious pleasure....