With that year expired the term for which he had been elected to the Slade Professorship, and in January 1873 he was re-elected. In his first three years he had given five courses of lectures designed to introduce an encyclopædic review and reconstruction of all he had to say upon art. Beginning with general principles, he had proceeded to their application in history, by tracing certain phases of Greek sculpture, and by contrasting the Greek and the Gothic spirit as shown in the treatment of landscape, from which he went on to the study of early engraving. The application of his principles to theory was made in the course on Science and Art ("The Eagle's Nest"). Now, on his re-election, he proceeded to take up these two sides of his subject, and to illustrate this view of the right way to apply science to art, by a course on Birds, in Nature, Art and Mythology, and next year by a study of Alpine forms. The historical side was continued with lectures on Niccola Pisano and early Tuscan sculpture, and in 1874 with an important, though unpublished, course on Florentine Art. It is to this cycle of lectures that we must look for that matured Ruskinian theory of art which his early works do not reach; and which his writings between 1860 and 1870 do not touch. Though the Oxford lectures are only a fragment of what he ought to have done, they should be sufficient to a careful reader; though their expression is sometimes obscured by diffuse treatment, they contain the root of the matter, thought out for fifteen years since the close of the more brilliant, but less profound, period of "Modern Painters." The course on Birds[29] was given in the drawing school at the University Galleries. The room was not large enough for the numbers that crowded to hear Professor Ruskin, and each of these lectures, like the previous and the following courses, had to be repeated to a second audience. Great pains had been given to their preparation--much greater than the easy utterance and free treatment of his theme led his hearers to believe. For these lectures and their sequel, published as "Love's Meinie," he collected an enormous number of skins--to compare the plumage and wings of different species; for his work was with the _outside_ aspect and structure of birds, not with their anatomy. He had models made, as large as swords, of the different quill-feathers, to experiment on their action and resistance to the air. He got a valuable series of drawings by H.S. Marks, R.A., and made many careful and beautiful studies himself of feathers and of birds at the Zoological Gardens, and the British Museum; and after all, he had to conclude his work saying, "It has been throughout my trust that if death should write on these, 'What this man began to build, he was not able to finish,' God may also write on them, not in anger, but in aid, 'A stronger than he cometh.'" [Footnote 29: March 15, May 2 and 9; repeated March 19, May 5, and 12, 1873.]