Tolkien never gave a comprehensive explanation for his position, but Robert Stuart in his Tolkien, Race, and Racism in Middle-earth (2022) argues that Tolkien c

Tolkien never gave a comprehensive explanation for his position, but Robert Stuart in his Tolkien, Race, and Racism in Middle-earth (2022) argues that Tolkien consistently assumed that all the flesh-and-blood humanoids in Middle-earth were interbreedable.