Surnames in the Highlands
While surnames were used from quite an early date by the fine of the leading Highland clans - those most closely related to the chief - most Highlanders were identified until at least the 18th century, and often well into the 19th century, by a combination of patronymics and bynames or nicknames. Even when chiefs, church ministers, and government bureaucrats insisted on individuals appearing in written records with surnames it doesn't mean those so-identified used surnames in everyday life - or, and this is the important point, that their surnames were fixed. Lairds often adopted their wife's or their mother's surname when inheriting estates or chiefships through them. So it's said that many Morrisons are in fact MacDonalds by blood due to the marriage of a Morrison heiress many centuries ago; while the current MacLeod chief is the great-grandson of a Hubert Walter who in 1901 married the heiress of the last of the male MacLeods of Dunvegan. Even where estates and titles were not at stake husbands who chose to follow the chiefs of their wife's clan might choose her surname, or have descendants who made that choice when surnames became necessary, because they had become members by adoption of her clan. Ordinary clanspeople might have less choice in the matter when ambitious chiefs like Seaforth, Lovat, or Lochiel decreed that tenants must adopt their surnames. So Mathesons & MacAndies might become MacKenzies; Barrons & Lees became Frasers; and the names MacGillonie & MacMartin virtually disappeared as Camerons multiplied. Often, however, at least for the first generation or two, their original surnames may continue to appear in OPRs and Estate-Records as "aliases" - so it's always worth checking back in the records for clues to the origins of a particular branch of one of the greater clans (see the page on Septs).
The most dramatic instances of apparent multiple-surnames often occur as a result of the Englishing of awkward Gaelic names by English-speaking ministers or clerks. Famous examples in Argyll include MacLeays becoming Livingstones; MacIlvernocks becoming Grahams; MacCallums becoming Doves; and MacIlvoyles becoming Bells. While the choice of the English name sometimes appears to have been quite arbitrary (e.g. Livingstone for MacLeay), in others there is some apparent connection between the two names; as for instance with MacCallum, the Gaelic stem of which - Colm or Columb (the name of the famous Saint Columba) - is said to stand for the dove of peace. There's also a connection between MacIlvolyle and Bell which demonstrates just how tortured some of these derivations can be. MacIlvolyle is a phonetic form of MacGhillemhaoil (the Gaelic "mh" being pronounced like the English "v"), which is one of the two Gaelic names usually translated as MacMillan, and a 16th century genealogy of a family called Leny or de Lany (who were also MacMillans by descent) has the name as MacGilbile because "...mhaoil" was phonetically similar to "bhaoil". In this case therefore Bell is not a different English surname, it's just a simpler form of the Gaelic original. Since MacMillans are also found as Camerons (because the Lochaber branch were followers of Lochiel) and Buchanans (because an 18th century Buchanan historian claimed the MacMillans as a sept of his clan) there are documented examples of individual MacMillans appearing with at least three surnames (e.g. an early 19th century emigrant who married in Scotland as "Cameron", travelled across the Atlantic as "Buchanan", and lived in Canada as "MacMillan").
For information on individual surnames, see George F. Black, Surnames of Scotland (New York, 1946; Edinburgh, 1999 & 2004).