FORMS ALLIED TO AMMONITES, 217 
in the convolutions was to the spinous Ammonites an 
advantage which would be established by selection. If 
other palzontologists 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 
iG eet 
FIG. 21. 
such a degree by curtailment of development that the 
indication of the nature of the progenitors continually 
diminishes. But our theory necessarily leads to the 
conviction that the families within which we have as 
yet been able to compare Ontogenesis with Philogenesis, 
