THE DECADENCE OF AMMONITES 407 



bottom. But this very complication would militate against 

 the race retaining sufficient plasticity to vary effectively when 

 conditions so adverse to them prevailed during and at the close 

 of the Cretaceous. These conditions probably consisted in the 

 main of a marked alteration in the distribution of land and sea, 

 and a consequent radical change in currents — a change by which 

 littoral organisms would be more easily affected adversely than 

 free swimming creatures. 



