The Jils are from a planet with higher gravity than humans, and are larger, faster and stronger. They are also a little ahead of humans, as they left their planet before humans did. They are also *all* psychics. However, one of the (unstated) reasons that most of them really dislike humans is that the average Terran psychic is generally better than the average Jil psychic. Both groups have their outstanding ones and those that aren't so good, so sometimes a Jil psychic will be the better psychic, but more often the human will be the better one. However, there are a lot more Jil psychics than human ones, because only some humans are psychics, so the Jils have the advantage. And then, of course, you have the ones like Trevor and Mark Linley.
For a more complete explanation, read Slave Race. What you may not have realized is that Trevor is what is called a psychic power pack, a Terran psychic without the "control factor", whose psychic energy can't be sensed, and who is a carrier of the psychic gene. When paired with a functioning psychic (like Jason) whose mind is a perfect complement of his, the psychic can draw power from his power pack and use it to enhance his own. This is covered in much more detail in the other stories.
Nan
