Fortey holds the history of life on Earth in the palm of his hand. By drawing comparisons between existing organisms and plants and those preserved in fossils, he tells the story of evolution's successes and failures. Besides horseshoe crabs and velvet worms, meet the lungfish, the ginkgo tree, and the elusive tinamou, the most primitive bird alive, and know their struggle to persist - yes, thrive - through eons of time. He has full command of the subject both in the broad sweep and in the details. I appreciated his little asides on the current scientific literature suggesting that this or that consensus view might be wrong, or only partly correct (and thus signaling to the non-scientific reader that the body of science is ever-changing, always going forward by discussion and disagreement). The level of the book is about right: As a (physical) scientist myself I found it somewhat breezy (but I wasn't looking for technical writing), but the layperson should find it just right.
In truth, I rate this book a low 5, as the writing doesn't approach the luminous transcendence of that of nature writers such as Loren Eisley or even John J. Rowlands ("Cache Lake Country"). Fortey is a materialist (as most scientists are), showing wonder in nature as known solely through scientific inquiry. But I believe science cannot answer all questions; at the end of the day there is still mystery and awe in the realization that the whole is not the sum of the parts. The last two chapters pick up a bit on the "big themes", but Fortey also soft-pedals political issues, such as our own role as the cause of a current great extinction, and of climate change. That being said, "Horseshoe Crabs" represents the best in scientific writing in our generation.