Next to the table of contents there is a simple geological time scale, with emphasis on the Cenozoic, there are 5 chapters in the book and they correspond to each of the five major subdivisions of that era. Each contains a short list of the representative mammals of the period in question, with brief descriptions, usually the height, number of toes and inferred diet. There are a couple of places where ancestor/descendant relationships are mentioned alongside full-page illustrated diagrams Short sections on rhino, horse and elephant evolution. Chris Forsey's restorations are nice. Each chapter opens with a color spread and the next page clearly points out the names (and pronounciation) of each animal that appears in the spread.
Two pages on Darwin's Beagle voyage are added near the end of the book. There is little mention of Mesozoic mammals, since these were not "mighty", of course. I would recommend it for junior high-schoolers and under.