I suppose that actually the chief difficulties I have involved myself in are scientific and biological; which worry me just as much as the theological and metaphysical (though you do not seem to mind them so much). Elves and Men are evidently in biological terms one race, or they could not breed and produce fertile offspring; even as a rare event : there are 2 cases only in my legends of such unions, and they are merged in the descendants of Eärendil. But since some have held that the rate of longevity is a biological characteristic, within limits of variation, you could not have Elves in a sense 'immortal'; not eternal, but not dying by 'old age'; and Men mortal, more or less as they now seem to be in the Primary World; and yet sufficiently akin. I might answer that this 'biology' is only a theory, that modern 'gerontology', or whatever they call it, finds 'ageing' rather more mysterious, and less clearly inevitable in bodies of human structure. But I should actually answer: I do not care. This is a biological dictum in my imaginary world. It is only (as yet) an incompletely imagined world, a rudimentary 'secondary'; but if it pleased the Creator to give it (in a corrected form) Reality on any plane, then you would just have to enter it and begin studying its different biology, that is all.