ARRESTED REPTILIAN EVOLUTION 233 



tion has exerted less influence on reptilian evolution during the 

 Age of Mammals than previously. Thus we shut out four out 

 of five factors, namely, physical environment, individual habit 

 and development, life environment, and selection as reasonable 

 causes of the relative arrest of evolution among the reptiles. 



Consequently the causes of the arrest of evolution among 

 the Reptilia appear to lie in the internal heredity-chromatin, 

 i. e., to be due to a slowing down of physicochemical inter- 

 actions, to a reduced activity of the chemical messengers which 

 theoretically are among the causes of rapid evolution. 



The inertia witnessed in the entire body form of static or per- 

 sistent types is also found to occur in certain single characters 

 of the individual. Recurring to the view that evolution is in 

 part the sum of the acceleration, balance, or retardation of 

 the velocity of single characters, the five surviving orders of 

 the reptiles appear to represent organisms in which the greater 

 number of characters lost their velocity at the close of the 

 Age of Reptiles, and consequently the order as a whole re- 

 mained relatively static. 



