766 THE EQUIANGULAR SPIRAL [ch. 



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 



M^iZ 



Fig. 363. A spiral foraminifer {Pnlvinulina), to shew how each successive chanibei 

 continues the symmetry of, or constitutes a gnomon to, the rest of the structure. 



of gnomons that we nmst never expect to find the logarithmic spiral 

 manifested in a structure whose parts are simultaneously produced, 

 as for instance in the maririn of a leaf, or among the many curves 

 that make the contour of a fish. But we most look for it wherever 

 , - ' the organism retains, and still presents 



at a single view, the successive phases of 

 preceding growth : the successive magni- 

 tudes attained, the successive outhnes 

 occui)ied, as growth pursued the even 

 tenor of its way. And it follows from 

 this that it is in the hard parts of 

 organisms, and not the soft, fleshy, 

 actively growing parts, that this spiral 

 is commonly and characteristically 

 found: not in the fresh mobile tisssue 

 whose form is constrained merely by 

 the active forces of the moment; but 

 in things like shell and tusk, and horn and claw, visibly composed 



Fig. 364. Another spiral fora-. 

 minifer, Cristellaria. 



