Among the differences it may be remarked that the figure-subjects on Irish crosses almost invariably refer to the scenes of the Passion, or are taken from the Bible. On the Scotch stones, they as constantly refer to battle or hunting incidents, or to what may be considered as events in civil life. The essential difference, however, is, that, with scarcely an exception, the Pictish stones bear some of those emblems which have proved such a puzzle to antiquaries. The so-called broken sceptre, the brooch, and the altar, are seen in the Aberlemmo stone; but in earlier examples they are far more important and infinitely various.[313] It may also be worthy of remark that the only two real round towers out of Ireland adorn the two Pictish capitals of Brechin and Abernethy. All this points to a difference that can well make us understand why St. Columba should have required an interpreter in speaking to the Picts;[314] but also to a resemblance that would lead us to understand that the cemetery at Clava was the counterpart of that on the banks of the Boyne, with the same relative degree of magnificence as the Kings of Inverness bore to those of Tara; and if we do not find similar tumuli at Brechin or Abernethy, it must be that the kings of these provinces--if there were any--were converted to Christianity before they adopted this mode of burial. It may be suggested that, as Maes-Howe is certainly the lineal descendant of the monuments on the Boyne, it too must be a Celtic or Pictish tomb. For the reasons, however, given above, such a theory seems wholly untenable; but thus much may be granted, that such a tomb would probably not have been erected, even by a Northman, in a country where there was not an underlying Celtic or Pictish population. [Illustration: 96. Cat Stone, Kirkliston.]