The authors of the excellent Because of a Tree and many adult natural science books have once again managed to develop a great deal of complex and interrelated scientific material around the simple core of an isolated phenomenon. The horseshoe crab is a commonplace sight at beaches along the Atlantic coast. Almost identical types have been swimming around in the briny for the last 500 million years. From the point of view of their non-evolution, the authors lead into an unusual discussion of how and why adaptation occurs and what special attributes enable horseshoe crab to adjust so well. This cross-sectional view of the sciences dips into paleontology, geology, anatomy, zoology, and ecology to form an intelligent readable narrative.