Structure of the Earth decay of radioactive elements, and freezing of the inner core. Structure of the Earth The internal structure of the Earth is layered in spherical shells: an outer silicate solid crust, a highly viscous asthenosphere and mantle, a liquid outer core that is much less viscous than the mantle, and a solid inner core. Scientific understanding of the internal structure of the Earth is based on observations of topography and bathymetry, observations of rock in outcrop, samples brought to the surface from greater depths by volcanoes or volcanic activity, analysis of the seismic waves that pass through the Earth, measurements of
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