[Legolas]

From Letter No. 153, the oft-quoted letter that addresses several points, though it was never sent - marked "taking myself too seriously" and set aside.

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.
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.

I've boldfaced the statements that address the biological relatedness of Men and Elves and italicized those statements that state the purpose and provide 'justification' for what may appear to some as 'wrong' or at least inconsistent with our world. The main problem with Elves and Men being the same race Mr. Hastings must have presented (or someone else whom Tolkien discussed the matter with) was the difference in longevity - stating that for one to be immortal and one mortal means they are too different to be called the same race (though I disagree with this and find Tolkien's concept to be okay even by our world).

In the italicized comments, you can read for yourself why he considered them to be so, but also Tolkien points out that this is indeed a work of fiction and not reality, and in such a case, the biology of his world can't be expected to match up the biology of our world. This difference means that there may be some inconsistencies that puzzle us at first glance. It's a part of his world though, where new and different biorealities may apply.

I wouldn't call them inconsistencies (except for immortality and Elves themselves since both are fictitious, obviously). I used 'biorealities' instead of biological information because biological implies biology (study of life) and such information is just knowledge through observation - only knowing as far as you can observe and understand yourself, which isn't always the whole picture. While Arda was supposed to flow into our world, looking at it through our eyes isn't going to work all of the time. "Species" is just part of a method of classification developed relatively recently and used by science almost universally. Every guideline we have for considering organisms to be of the same species, genus, family, order, class, phlyum, or kingdom is based on the organisms that we've been able to study, whether dead or alive. Difference in longevity/mortality has never been a complication in our world because there is no such thing as (and nothing similar to) the Elves' immortality. We have no elves to study.

So the question is, are Elves and Men the same race? Well, does the difference in longevity mean enough to consider them separate races? Would immortality set organisms apart at the kingdom level? Would there be an 'immortalia' kingdom? Or at a different level of the tree? Tolkien as Middle-earth's all-knowing creator says they are of the same race, so they must be part of the same species - means the division must be subspecies or however you want to break it down further.

In the quote, when Tolkien says 'race,' he is intending 'species' as we know it, since the term 'race' has come to classify more in terms of a broad ethinc background (Hispanic, Caucasian, etc.) rather than a higher division/difference like 'species' where the ability to produce fertile offspring would be in question.