Tattoos, Butts, and Half Breeds...

[Wyldkat99]

http://forum.square-enix.com/ffxiv/t...l=1#post490835
The races are intermixable, but it's not something that's totally accepted. I don't remember where it was and it might even been on the old 1.0 forum, but there was a lengthy discussion on the Races.
I seem to recall at some point there was a statement that all the spoken races were technically human, with the various differences caused by aetheric and other magical influences. While that might have been revised since ARR.

And if you think that sounds weird I point you to the various dragons we have in HW that are not only all part of the same race but technically part of the same family that all look drastically different.