586 THE LOGARITHMIC SPIRAL [ch. xi 



said, ex sola nascuntur diversitate gyrationum ; and which accord- 

 ingly are seen to have their origin in differences of rate, or of 

 magnitude, and so to be, essentially, neither more nor less than 

 differences of degree. 



In nature, we find these forms presenting themselves with 

 but little relation to the character of the creature by which they 

 are produced. Spiral forms of certain particular kinds are common 

 to Gastropods and to Cephalopods, and to diverse families of 

 each; while outside the class of molluscs altogether, among the 

 Foraminifera and among the worms (as in Spirorbis, Spirographs, 

 and in the Dentalium-like shell of Ditrupa), we again meet with 

 /similar and corresponding forms. 



Again, we find the same forms, or forms which (save for external 

 ornament) are mathematically identical, repeating themselves in 

 all periods of the world's geological history ; and, irrespective of 

 climate or local conditions, we see them mixed up, one with 

 another, in the depths and on the shores of every sea. It is hard 

 indeed (to my mind) to see where Natural Selection necessarily 

 enters in, or to admit that it has had any share whatsoever in the 

 production of these varied conformations. Unless indeed we use 

 the term Natural Selection in a sense so wide as to deprive it of 

 any purely biological significance; and so recognise as a sort of 

 natural selection whatsoever nexus of causes sufiices to differ- 

 entiate between the likely and the unlikely, the scarce and the 

 frequent, the easy and the hard : and leads accordingly, under 

 the pecuUar conditions, limitations and restraints which we call 

 "ordinary circumstances," one type of crystal, one form of cloud, 

 one chemical compound, to be of frequent occurrence and another 

 to be rare. 



