[Lord Gothmog]

Originally Posted by Galendor

One odd thing about Tolkien's Elves is that they can mate with humans, and produce viable offspring that can then supposedly successfully mate with other elves or humans. Is the last bit true - does Tolkien's work ever indicate that the offspring of a Elf-human sexual union (such as Aragorn and Arwen's child) can then go on to have children with another human or Elf? I don't know the answer, I'm asking.

If so, then Elves and humans are THE SAME biological species, and it is strange that the phenotype (outward appearance and abilities) would be so different between these two "strains" of the same species. For example, Elves are immortal whereas humans are mortal, Elves have better eyesight than humans, and so on.

Tolkien did give much thought to this problem and here is his answer found in Tolkien's Letters: 153 to Peter Hastings

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.1 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.

But as it is — though it seems to have grown out of hand, so that parts seem (to me) rather revealed through me than by me – its purpose is still largely literary (and, if you don't boggle at the term, didactic). Elves and Men are represented as biologically akin in this 'history', because Elves are certain aspects of Men and their talents and desires, incarnated in my little world. They have certain freedoms and powers we should like to have, and the beauty and peril and sorrow of the possession of these things is exhibited in them. ....