Structure of the Earth the crust, upper mantle, lower mantle, outer core, and inner core. The geologic component layers of Earth are at the following depths below the surface: The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes. The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers. The changes in seismic velocity between different layers causes refraction owing to Snell's law, like light bending as it passes through a prism. Likewise, reflections are caused by a large
How the Snake Lost Its Legs a single chapter providing "An evo-devo bestiary," a long list of stories, such as "How the turtle got its shell", "How the vampire bat reinvented running", "How the quetzal got its crest", and "How the firefly got its flashlight". These are "Just So" only in name, since each one is reliably cited to recent research rather than an author's whimsy. Since by this point the reader has been introduced to the core elements of the evo-devo gene toolkit, Held makes each section brief, 50 stories in 32 pages, and minimally technical: he discusses what the evo-devo system achieves in terms