FORMS ALLIED TO AMMONITES. 



217 



in the convolutions was to the spinous Ammonites an 

 advantage which would be established by selection. If 

 other palc-Eontologists consider the fluctuations of form 

 accompanying the relaxation of the closed spiral as 

 evincing the decline of the group, no contradiction 

 seems to be implied, for what was originally used as an 

 advantage by natural selection, proved injurious in its 

 consequences. 



As we have seen, the earliest states are obliterated to 



Fig. 21. 



such a degree by curtailment of development that the 

 indication of the nature of the progenitors continuall\' 

 diminishes. But our theory necessariU' leads to the 

 conviction that the families within which we have as 

 yet been able to compare Ontogenesis with Philogenesis, 



