Knotwork was gradually evolved from the plait by introducing breaks at regular intervals during the Bizantino-Barbaro period (A.D. 600 to 800); and subsequent to this we find still more complicated forms of interlaced patterns were introduced, which I propose to call circular knotwork and triangular knotwork. The evidence gathered from dated examples of interlaced-work in Italy tends to show that there was a gradual advance in the elaboration of the patterns as time went on. Consequently the style could not have been borrowed _en bloc_ by Ireland from Italy, or _vice versâ_, at one time; but interlaced ornament must have been a prevalent form of decoration throughout the whole of the West of Europe, and the style advanced in all the different countries simultaneously, there being always a constant communication between Rome and the centres of religious activity abroad. Some races, like those in Great Britain, who appear to have had a special gift for inventing new patterns and combining them with a sense of artistic fitness, may have made more rapid strides than their neighbours and have influenced the development of the style in consequence, but that is all that can be said. [Illustration: KEY PATTERN, S. APOLLINARE NUOVO, RAVENNA] [Illustration: VINE SCROLLS, S. APOLLINARE NUOVO, RAVENNA] Two special peculiarities of the Italian interlaced-work, as compared with that in Great Britain, are the ornamenting of the interlaced bands with two incised lines instead of one, and the twisting together two bands at frequent intervals, thus— [Illustration] The latter feature, which is clearly Classical, occurs frequently in circular knotwork in this country, showing that circular knotwork is of Italian origin. The reason why interlaced-work is characteristic of early Christian decoration almost throughout the whole of Europe, whilst spirals, key-patterns, foliage, etc., are confined to particular limited areas, I believe to be partly because the number of distinct patterns that can be produced from interlaced-work is far greater than those which can be got from any other class of ornament. It is hardly necessary, perhaps, to enlarge much upon the subject of the foliage of the early Christian period in Great Britain. The scrolls with conventionalised bunches of grapes are no doubt descendants of the Classical vine; the involved birds, beasts, etc., being a later addition[445] of the Bizantino-Barbaro, or Italo-Bizantino periods. Foliage is unknown in the Pagan Saxon, Scandinavian, or Late-Celtic art, and the only other source it could have been derived from is Italian art. [445] Or a substitution of later forms for the Cupids, etc., of the Classical style.