Based at the Parisian Museum of Natural History, Cuvier was able to compare the fossil bones he dug from the quarries of Montmartre with those of animals alive today. Guided by the principle of correlation, that all the parts of an animal must cohere, and by analogy, with living species, Cuvier boldly reconstructed extinct creatures from the incomplete skeletons he unearthed. This process is described in his Essay on the Theory of the Earth.