There are obviously ethical problems with having Supernaturals with so much power and mother nature has put some checks and balances in place so magical creatures don’t abuse the advantages they have over humans.

1. Dilution of power – intermixing weakens the amount of power. It makes staying in communities with your own kind much more appealing and limits the populations.
2. For witches, every time you use your magic, a bit of it rebounds back to you. So doing bad things to people makes bad things happen to you. It’s a little like karma, except you know exactly what will happen.
3. A natural dislike for other Supernaturals. All castes are suspicious of each other. Keeps them from interbreeding (except in the case of Rae and Connor – urgh! But they’re a special case. She’s human. Yes still.)
4. All immortal creatures have major weaknesses. The Fae can barely reproduce with each other and procreating with anything else produces something other than Fae. Genies are locked away and must be freed. Vampires are allergic to sunlight.
5. Non-immortals have it pretty bad too: Werewolves are controlled by the moon. Witches (reference #2). Mermaids need to live in or near the ocean.

Plus, humans can become supernatural. You have vampires and werewolves that get turned. And witchcraft is a skill any human can learn (in theory).

List of Supernaturals:

1. Fae/Fairies
2. Jinn/Genies
3. Mermaids/Mermen
4. Vampires
5. Werewolves
6. Witches (Natural-born)