The evolutionary developmental biologist Lewis Held wrote that "The notion that aspects of anatomy can be explained by physical forces (like expansion cracking) was advocated ~ 100 years earlier in D'Arcy Thompson's 1917 "On Growth and Form" and in Theodore Cook's 1914 book "The Curves of Life". Over the intervening century, various traits have been proposed to arise "mechanically" rather than "genetically": brain convolutions, cartilage condensations, flower corrugations, tooth cusps, and fish otoliths. To this kooky list we can now add the crooked smile of the crocodile, or at least the cracked skin that surrounds it."