I addressed this issue in the book, though it'll never appear anywhere within the book.
Gora's Law does apply between Earth and Draconis. Both have virtually identical geologic histories and environmental conditions. There are animals on both planets that look almost identical, and could interchangably survive if moved to a similar ecosystem on the other planet.
This is an actual IRL scientific theory, btw, though the name of it escapes me. I remember reading it somewhere.
Gora's Law connects Earth and Draconis, but only so far. A Vulpar and fox are "interchangable" animals in their environments, but there are differences. Vulpars have two tails and have a lifespan of around 70 years, where foxes have one tail and live for only about 15 years. A Vulpar and a fox can't interbreed.
Interchangeable species between Earth and Draconis cannot, as a rule, interbreed. They are
similar, but not genetically compatible. The few that can interbreed produce sterile offspring that aren't viable for continuing a crossbred "merged species," like the mule offspring of a horse and a donkey.
There is only one mirror species combination between Earth and Draconis that can breed viable offspring: humans and Faey.
Why? Simple. The Faey aren't native to Draconis. They
think they are, and thanks to Gora's Law, they're just similar enough to
appear that they are, but they aren't. Faey and humans can breed because Faey and humans are [related]. The Faey are actually a branch race of humanity, and are from Earth. They're similar to the Neanderthal race (but prettier), a branch of the homo sapien line, but one that went extinct on Earth a very, very long time ago. So long ago that the fossil record of them is mistaken for other, closely related species.
They were moved to Draconis by one of the ancient races that are now long extinct, hundreds of thousands of years ago, to separate the Faey's ancestors from their human cousins. Even at that stage, the first vestiges of telepathic ability was starting to manifest itself in them, and the ancient race saw that and realized that this new offshoot of the proto-human line would be eradicated by its larger cousins. So they were moved. On Draconis they had no competition (Draconian simians had not yet reached the proto-sapien stage), they evolved into the modern Faey, and their telepathic abilities matured.
That's also why the humans and the Faey act similarly. They have the same biological instincts, the same drives, and the same violent tendancies.
There are some differences though, some cultural, some biological. Despite the way it looks, Faey, ah, "wantonness" isn't entirely cultural. Their loose attitude towards sex is actually based on their slightly different biological drives, an evolutionary alteration from humans based on their telepathic ability. Faey females didn't evolve needing to play "hard to get" to secure and keep a mate, since their telepathic powers formed a pair bond that kept their male with them. But the need to pair-bond is still the main driving force behind most of their activity, just like in human females. It's why Faey women search for a husband, but unlike human women, they'll go to much greater lengths to secure one, because males don't easily settle down. That's why Faey women so aggressive. It's not entirely because they're the dominant gender, it's because they are fulfilling the need to find a male to pair bond with. But, since they ARE the dominant gender, they're just like human men. They are the ones who have evolved with the biological aggression, not men, and they exhibit it in most ways a human male would. They have to be aggressive in order to secure a pair bond, to satisfy the biological need to mate and produce offspring. In Faey biology, the woman has to win the man, not the other way around. She does this any way she can, and sex is simply another tool in the arsenal to attain the objective, and a darn effective one at that. If it wasn't effective, they wouldn't have used it.
So they're not just sluts for the fun of it. Their sluttiness has grounds in Faey genetics.
It's also why Faey males are the ones that play "hard to get." They instinctively know that the pair bond will secure them to one woman until death, not able to freely pursue any woman, so they resist it to satisfy their genetic need to spread their genes to as many offspring by as many female mates as possible.
And since culture is often an end result of biology, that's why the Faey have developed such loose ideas about the physical act of sex. Faey females evolved using sex as a weapon against men to try to lure them into a pair-bond relationship, and males are just males no matter how you look at them, so the Faey never developed any kind of social stigma about it.
Truth be told, it's the same in OUR biological history. Go look at the real history of some of the ancient civilizations, like the Romans, and you'll see that our "modern" views on sex would seem downright draconian to them, almost inconceivable. The only thing that caused modern society to adopt our views on sex was the influence of religion...an influence that Faey religious practices never adopted.
Thus Jason and Tim's "conversion" to Faey views on the matter. It's innately biological, it was just a matter of cultural upbringing. Since Jason is so much more disciplined than Tim, he clung to his cultural outlook much more strongly. It took Symone and Jyslin a long time to wear him down and make him see things their way.
