Structure of the Earth easily than does the upper mantle (chemical changes within the mantle may also be important). The viscosity of the mantle ranges between 10 and 10 Pa·s, depending on depth. In comparison, the viscosity of water is approximately 10 Pa·s and that of pitch is 10 Pa·s. The source of heat that drives plate tectonics is the primordial heat left over from the planet’s formation as well as the radioactive decay of uranium, thorium, and potassium in Earth’s crust and mantle. The average density of Earth is . Because the average density of surface material is only around , we must
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