This isn't a horror story, although body horror is certainly present, but something about it feels like it is. Or maybe it's just me -- stuck inside a human body imagining the body shifting and changing into one of the detailed and grotesque illustrations within the text. This book takes place over billions of years, there is no one character or mind we are stuck in, we never feel their personal jolts of terror at how the very nature of their species has shifted. They don't feel that terror, because they themselves are not changing. Evolution takes place over millions of years. An ape does not fear becoming a man, because it will live and die an ape, and it's great-great-great-etc grandchildren will simply awaken as a man, having never been anything else. But I still found myself afraid. I think speculative evolution is easy when you imagine what it would take for a species to become more human (what kind of a society would intelligent avians have? How would aquatic species communicate or adapt for speech?) but difficult when you are forced to confront humans becoming more animal.