While this novel is based on an intriguing concept, millions of parallel Earth's that are subtly different (though Homo Sapiens has not evolved on any of them), it never quite came together for me. The best parts were those that dealt with the degeneration of civilisation as millions of people discovered that with the aid of a simple piece of engineering, they can move (step) to another Earth. The spirit of adventure, exploration and the desire for a better life undermine the viability of many countries. In turn the minority that cannot step let their anger and jealousy flourish.
But the rest of the story, based around a deep exploration trip into far-flung parallel Earths never grabbed me. The central story of why some of the non-Homo Sapien species were fleeing away from a particular parallel-Earth seemed flimsy, and our story's heroes spent so little time on some of the more intriguing other-worlds that I was constantly disappointed. All in all it was quite disappointing.