Picts by the name of Pict may be historically incorrect, but they are clearly a seperate people. The Romans DO state they were shorter, and while I'll need to find the exact qoutes, Michael Lynch, a very prominent Scottish historian (his finest composition is the Oxford Companion to Scottish History, an excellent source), states so, and the Irish and the Dal Riatans, in later periods, describe the Pict people as not being of the Gaelic race. However, they called the various Welsh, the Strathclyders, the people they called Caledonians, and most of the southern British tribes of Celts Gaels, but they likened the Picts to the Cruithe. It is an accepted fact in Irish history, that the Tuatha Da Danann were Cruithe, and they were also short black haired people, they were described by Saint Patrick, who dealt with the last Irish Cruithe in the north of Ireland as "a people totally unlike the Gaels, but claiming of proper rights to the island, stating they were before the Gaels". The Irish didn't argue that they weren't, either. Rather, the idea that these people were there first is a key note in the earliest of Irish history.
Saint Donan, who went on mission to the Picts, called them Celts, but said of them "They are shorter than the Gaels, but have longer heads, and whiter skin" in his letter to the monastary at Iona (shortly before he and his 52 missionaries were slaughtered at Eigg). The letter also includes references to their religion (worship of rocks, trees, rivers, and sometimes, bastardized versions of British gods, though the former, animism, was generally more popular). Donan, like Patrick (a Cumbrian), declared himself a member of the race he was trying to convert, but was of Gaelic extraction (Donan was from Dal Riata), and, even if the Picts were Celts, was not of the same race (as if the Picts were Celts, they were Brythonic, not Goedelic). Supposedely, Saint Columba would not be anamchara (soul-friend) with Saint Donan, because he was 'of the Pict race', who Columba thought of as irredeemably pagan, and actually likened them to monsters, and used their PHYSICAL APPEARANCE (notably, their size, their gait, as well as non-natural things, such as hair styles and tattoos) to spread propaganda that they were in fact the children of demons during his early missions, before changing his mind after Donan's martyrdom. only after Donan's marytrdom did Columba speak with Brude, the Pict king, who he'd originally called 'a little bile spat up from hell'.
A common theme in all of the statements of the interactors with the Pict regions is that they were a shorter people (not necessarily SHORT, but compared to their neighbors, they were generally shorter). The Gaelic races all say they (the Cruithe) were there before them, and that they weren't related to them remotely (until the assimilations of the Picts into the people of the kingdom of Dal Riata, then Alba).
The sources I'm using, I know, are post Roman, but they talk a lot about pre-Roman eras for the Picts, and provide a clear description of the northernmost as being a different people. The southern Picts would surely be interbred heavily with actual Britons, but I think it would be foolish to assume that the Picts are purely Celtic, based only on Roman sources, when the Romans did not interact that heavily with them. They fought them, and saw them, but they did not engage in long diplomacy with them, nor study them as Saint Donan did. Unless you're willing to believe a massive invasion of totally unrelated people flooded into Scotland without anyone, British or otherwise noticing, or that by some genetic anomaly, the northernmost Britons looked totally different than their southern cousins, there is no way the Picts can be of the same race.
Bookmarks