in STABILITY OF FORMS 147 



lasting only for a time, and all of them 

 in their turn superseded by still newer 

 forms. There is no sign of mixture, or 

 of confusion or of infinitesimal or of in- 

 determinate variations. These " Medals 

 of Creation " are all, each of them, struck 

 by a new die which never failed to im- 

 press itself on the plastic materials of this 

 truly creative work. There is nothing 

 more instructive than to place a series 

 of these new species, such as the Ammon- 

 ites, on a table side by side. The perfect 

 regularity and beauty of each new 

 pattern of shell, and the fixity of it so 

 long as it existed at all, are features as 

 striking as they are obvious. 



There is one suggestion which has 

 been made in order to meet these strange 

 phenomena, which has always seemed 

 to me to be more plausible than any 

 other, and to come much nearer than 

 any other to the historic facts. It was 



