Not sure if anyone still following this up but I am a direct descendent of James Mearns and Margaret Aird through their Grandson William (1836 - 1924) and his wife Christina H Purvis (1835 - 1909). My information is that James Mearns and Margaret Aird were not married when their son also James (1806 - 1867) was born. I think James married a woman by the name of Jean Russell in 1809 and had another family.
I have traced the male line back to 1703. James was born 11/12/1787 to William Mearns and Jean Tennant (m 22/11/1777). William (6/3/1747) is the son of Andrew Mairns (b 3/10/03 - listed on birth register as Mearns) and his father was John Mearns of Syke in Ayrshire, his mother was Bessie Buie. They married in 1702. The only other child of John and Bessie I can trace is a George (29/10/1711 - 17/6/1779). I am interested in tracking the other descendents of Andrew and George. They had several children and lived in Sorn, Ochiltree and Catrine.