We see that the successive chambers of a spiral Nautilus or of a straigt Orthoceras, each whorl or part of whorl of a periwinkle or other gastropod, each new increment of the operculum of a gastropod, each additional increment of an elephant’s tusk, or each new chamber of a spiral foraminifer, has its leading characteristic at once described and its form so far explained by the simple statement that it constitutes a gnomon to the whole previously existing structure. And herein lies the explanation of that 'time-element' in the development of organic spirals of which we have spoken already; for it follows as a simple corollary to this theory of gnomons that we must never expect to find the logarithmic spiral manifested in a structure whose parts are simultaneously produced, as for instance in the margin of a leaf, or among the many curves that make the contour of a fish. But we must look for it wherever the organism retains, and still presents at a single view, the successive phases of preceding growth: the successive magnitudes attained, the successive outlines occupied, as growth pursued the even tenor of its way.
