And so one thing leads to another...
The following quote from Keith Dewar's book on the Dewars of Glengarry. So, although James' origins are still a mystery, I'll leave that to someone else to discover.
Evander John Dewar son of Angus Murdoch Dewar
Evander John Dewar (b. Jul 03 1816) in Galder, Glenelg; (d. Sep 13 1885) at Lot 1 Con. 03, Lochiel. Same address as Duncan Edward Dewar. Then moved to Lot 26 Con. 6 Lochiel. First married Flora Fraser (b. 1802) and (d. 1841); second marriage on (m. Jan 30 1845) at East Hawkesbury to Margaret MacNaughton (b. Nov 16 1826) (d. Oct 30 1909); buried at east church Kirkhill; daughter of Alex MacNaughton and Ann Ferguson of Cote St George. In this family burial plot there is a listing for a James Dewar (d. 1844) at 44 years of age. This could have been Evander’s brother or a wrong date for a son. Lot 01 Con. 03 Lochiel, Crown grant to Murdoch Dewar and Evander Dewar in Jul 27 1847. Evander willed this farm to his son Angus Dewar in Oct 1885; Angus sold it in 1909.