Walter Coningsby Erskine's claim
to the Earldom of Mar
The deeply unprepossessing 12th Earl of Kellie was the sort of chap for whom just the one Earldom was intolerably insufficient. He looks like he regularly raided the fridge at night, and certainly took insufficient exercise.
It might be helpful to include some idea of the (English) legal principles of hereditary succession at this point.
In English law, heirs of the body is the principle that certain types of property pass to a descendant of the original holder, recipient or grantee according to a fixed order of kinship. Upon the death of the grantee, a designated inheritance such as a parcel of land, a peerage, or a monarchy, passes automatically to that living, legitimate, natural descendant of the grantee who is most senior in descent according to primogeniture, males being preferred, however, over their sisters regardless of relative age; and thereafter the property continues to pass to subsequent descendants of the grantee, according to the same formula, upon the death of each subsequent heir.
Collateral kin, who share some or all of the grantee's ancestry, but do not directly descend from the grantee, may not inherit. When there are no more heirs of the body, the terms of the original grant are expired, and the property becomes extinct (e.g. peerage), or some other criterion for allocating the property to a new possessor must be applied. If the original grant stipulated an alternative formula for succession upon exhaustion of heirs, that formula is immediately applicable. Thus, if a peerage is granted to "heirs of the body of John Smith, failing which, to heirs general", the title would pass to a descendant of John Smith's sibling when all of John Smith's descendants die out.
Thus property settled upon someone and the heirs of their body – whether male, female, or generally – will pass to children, grandchildren and so on, but not to nephews of the grantee, his or her sisters, uncles and their descendants.
There are other kinds of formulae for inheritance than heirs of the body, such as heirs male, heirs of the line, heirs portioners, heirs general, etc.
Heir male is a genealogical term which specifically means "senior male by masculine primogeniture in the legitimate descent of an individual"
Wikipedia is far from being comprehensive or infallible, and if we examine the family tree of Charles Erskine 8th Baronet of Cambo and 8th Earl of Kellie, we can see much wider degrees of heritability being applied, both in his own case and that of his uncles (who succeeded him as 9th and 10th Earls of Kellie. I think heirs-male must simply mean males of a common ancestry (no matter how far back) with the "grantee".
Whether heirs-general also includes females collateral with the grantee, Ah Jest Don't Know.
As we can see from the family tree, Walter Coningsby Erskine had been a cousin (through the male line) and heir male of the deceased Henry Francis Miller Erskine, 9th Earl of Mar, whereas John Francis Erskine Goodeve-Erskine had been a nephew (though the female line) and heir general.
The legal arguments raged for 15 years, reminiscent of Jarndyce v Jardyce, or Tweedledum v Tweedledee, based more on metaphysics and the interpretation of ancient power-politics than common sense.
They ended up in 1885 by Walter Coningsby Erskine's son being legally recognised as the Earl of Mar of the 1565 creation, and John Goodeve-Erskine being acknowledged as the Earl of Mar of the 1404 creation.
And at this point we can tiptoe away.