OF COURSE my dragons are evolved! What else? Genetically engineered like the ones of Anne McCaffrey? And then forgotten about the origin, so after two books we get from a SF into a fantasy realm? Maybe I'm just jealous... Why can't I do a similar thing without being told by half the readers "this isn't SF" and the other half "this isn't fantasy"?

And what you both said - SF readers not wanting "fantasy" messing things up and vice versa - especially vice versa - that's the whole problem. I bet there are people like me out there, who don't care if it fits some well-defined category. As long as the strange world is consistent within itself! I just can't seem to reach them...
At least, *my* science-fiction story doesn't have magic disguised as a "force of nature"... ;-)

A similar question: how do you determine for which age the book is? I've thought about a few factors:
- the age of the main character. Seems to be a huge factor. A story of mine, which I thought would be ideal for 12-14 years old, was rejected on grounds that "kids don't want to read about grown-ups" - the main was a 50-something adventurer. I thought the plot would be too light for adults.
- themes. I guess you don't want rape or abuse to get into a youth book. But does that mean that EVERY adult book needs depressing themes? I don't want to write about depressing things. There are enough of that already in real life, I don't want to repeat them in a book.
- sex. Does inserting a bare boob into a child's story make it an adult one? Far too many books in the adult section of our library were like that. The story far too childish and repetitive, and screeching to a stop once in a while to describe a sex scene. Hate this.

Edit. Thought about something. Star Wars and the Force which looks more like fantasy-style magic than like an unknown force of nature (reacts on emotion, turns you evil if you don't follow the rules); The Matrix with paranormal abilities you've got to achieve in the same way as if there weren't computers at the origin, and a prophecy about a Chosen One; ...
I guess you can market anything as SF if you only adapt the vocabulary. But Heaven help you if you let the people in your magic fantasy land discover and adhere to the scientific principle without making them lose the magic.