Charles Erskine's claim to the Earldom of Kellie
Given the abysmal reluctance of the Earls of Kellie to find a nice girl of good parentage to get cracking with (after the preliminaries of Kellieriage, of course – which seems to have been another insuperable hurdle) it was virtually inevitable that the line was going to peter out. As it did, twice.
At the death of Archibald 7th Earl of Kellie in 1795, there were no surviving Erskine males on the Kellie side, and but three Erskine males on the Cambo side: Charles the 8th Baronet of Cambo (1765 – 1799), and his uncles Thomas (1745 – 1828) and Methven (1750 – 1829).
The prevailing rules of kinship and primogeniture were evidently that the son of a deceased father outranked the younger brother(s) of that father. So Charles gained the Earldom in addition to his Baronetcy. Thomas and Methven also benefitted in due course, as each in turn inherited the Earldom.
You might like to see this displayed diagrammatically, and check that Charles was indeed Archibald's 3rd cousin once removed, and that Thomas and then Methven were indeed next in line after him.
And after Methven, then who?