The theory of the evolution of Celtic knotwork out of plaitwork (as explained on pages 257 to 278) is entirely original, and, simple as it appears when explained, took me quite twenty years to think out whilst classifying the patterns that occur on the early Christian monuments of Scotland, England, Wales, and Ireland, nearly all of which I have examined personally. No illustrations are given of the pages of the Celtic illuminated MSS. on account of the difficulty of making satisfactory reproductions of them on a small scale. I have thought it better to refer the reader either to the MSS. themselves or to the _Publications of the Palæographical Society_ and Professor J. O. Westwood’s _Miniatures of the Anglo-Saxon and Irish Manuscripts_. A large number of photographs of Late-Celtic metalwork in the British Museum have been specially taken for this work by Mr. H. Oldland, with the kind permission of Mr. C. H. Read, F.S.A. I am indebted to the Rev. Canon W. Bazeley for obtaining a photograph of the Birdlip mirror in the Gloucester Museum, and to Mr. George for the loan of Sir H. Dryden’s drawings of the Hunsbury sword-sheath in the Northampton Museum. Mr. George Coffey, M.R.I.A., of the Museum of the Royal Irish Academy, has also from time to time been good enough to assist me in various ways. The photographs of the cast of the Nigg cross were taken by Messrs. M. and T. Scott, of Edinburgh, for Mr. D. J. Vallance, the curator of the Museum of Science and Art at Edinburgh. For the use of electrotypes of blocks I have to give my best thanks to the Society of Antiquaries of London,[1] the Society of Antiquaries of Scotland,[2] the Royal Society of Antiquaries of Ireland,[3] the Royal Irish Academy,[4] the Royal Archæological Institute,[5] the Cambrian Archæological Association,[6] the Somersetshire Archæological Society,[7] and the publishers of the _Antiquary_,[8] the _Reliquary_,[9] and the _Illustrated Archæologist_.[10] Plates XXVI., XXIX., XXXV., XXXVI., and XXXVII. are from the series of photographs taken by Mr. W. G. Moore, of Upper Sackville Street, Dublin, for the Royal Irish Academy. [1] Blocks on pp. 112, 123, 152, 153, 166, 167, 168. [2] Blocks on pp. 154, 169, 183, 276 to 278, 274 to 288. [3] Blocks on pp. 150, 221. [4] Blocks on pp. 39, 40, 45, 47. [5] Blocks on pp. 119, 120, 131, 147. [6] Blocks on pp. 27 to 36, 181, 185, 187, 189, 191, 259 to 269, 271 to 273, 283. [7] Blocks on p. 142. [8] Blocks on pp. 147, 161. [9] Blocks on pp. 104. [10] Plate XXXIV. CELTIC ART IN PAGAN AND CHRISTIAN TIMES CHAPTER I THE CONTINENTAL CELTS AND HOW THEY CAME TO BRITAIN THE CELTS A BRANCH OF THE ARYAN FAMILY OF NATIONS