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 the butterfly, the snake, and the cheetah. Each consists of three to eight sections named in the style of Rudyard Kipling's "Just So Stories", with titles like "How the butterfly got its spots" or "How the snake elongated its body", though a couple of sections use "Why?" rather than "How?", as in "Why the fly twirls his penis" (it rotates during embryonic development under the control of different genes). In these chapters, Held explains the mechanics of evolutionary developmental biology, complete with accounts of what genes such as hox, hedgehog, and engrailed do to shape bodies. The third part is