... example, the small imperfections or bumps on a shell) are averaged out. To explain how the form of the shell arises, there is no point in trying to explain all the hide bumps, since these will ... or the right), but this instability sets in only when the speed falls below a certain threshold. Two common aspects of pattern- forming instabilities are that they involve symmetry-breaking (in ... found in ancient reef environments around the world. Ever since these curious, spongy structures were discovered in the nineteenth century, their origin has been disputed. The prevailing interpretation...