A thought About Butters

[Arjan]

Yes, but the line of the Kings of Gondor became diluted over time by marriage to normal people.  So not so direct, and over time though they still lived a bit longer than ordinary humans, they were diminished.  Marriages among the men of the North that Aragorn came from, did not and though there were way fewer of them, they kept many of the characteristics of original men of Numenor. 
It gets complicated,  Tolkien mapped out extensive family trees, and when Legolas said "the line of Luthien would never fail," it was Aragorn's line he was talking about and not the Stewards of Gondor.

Which means the blood of normal people got enriched by that of the line of the kings of Gondor. That is how these things work.

[Mira]

Which means the blood of normal people got enriched by that of the line of the kings of Gondor. That is how these things work.

Exactly, they all carried some of the genes, also there were some that did marry elves, though that was rare.  But returning to the real world, lots of kings fathered children outside of marriage, so while bastards usually didn't inherit a throne, they still were of that king's bloodline.