As the world changes and animals become the dominant civilization, the book examines both religion and the ideologies that can replace it. In the end, Mort(e) learns that love must be stronger than either religion or rationality.
There is much to like in this book, but also many ways in which it falls short. It covers a long time frame and sometimes moves so quickly that it's difficult to remain engaged in events. The characters are not especially deep, but that's often the case when it comes to SF of ideas. And I was bothered by the fact that when Mort(e) was finally reunited with Sheba by the ants, it turned out that she hadn't changed and was still a pet. Part of it may have been the chance to give him the choice about his own fate, a sort of opportunity to go back to Eden and innocence or stay in the new world and only then bring her with him, a decision that couldn't have been completely in his hands if she'd already changed. But she still hadn't finished the change at the point when the book actually ended, keeping her an abstract ideal/reward for him without ever showing what she turned out to be like as a full "person," so to speak. Still, it was an interesting book, and I am glad that I read it.