What do your Elven eyes see?

[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.