ENVIRONMENTAL CHANGES 137 



ing which vary in extent up to the submergence of 4,000,000 

 square miles of surface. 



Each of these changes, which by some geologists are be- 

 lieved to be cyclic, included long epochs especially favorable 

 to certain forms of life, resulting in the majority of cases in 

 high specialization like that of the sea-scorpions (eurypterids) 

 followed by more or less sudden extinction. In the oceans the 

 life most directly influenced was that of the lime-secreting 

 organisms which resulted in maximum and minimum periods 

 of limestone formation (oblique lines) by algae, pelagic fora- 

 minifera, and corals. On land there were two greater (Car- 

 boniferous, Upper Cretaceous) and several lesser periods of 

 coal formation. 



Changes of environment play so large and conspicuous a 

 part in the selection and elimination of the invertebrates that 

 the assertion is often made that environment is the cause of 

 evolution, a statement only partly consistent with our funda- 

 mental biologic law, which finds that the causes of evolution 

 lie within the four complexes of action, reaction, and inter- 

 action (see p. 21). 



Perrin Smith, who has made a most exhaustive analysis of 

 the evolution of the cephalopod molluscs and especially of 

 the Triassic ammonites, observes that the evolution of form 

 continues uninterruptedly, even where there is no evidence 

 whatever of environmental change. Conversely, environmen- 

 tal change does not necessarily induce evolution — for exam- 

 ple, during the Age of Mammals, although the mammals de- 

 veloped an infinite variety of widely divergent forms, the rep- 

 tiles (p. 231) show very little change. 



