ARRESTED REPTILIAN EVOLUTION 231 



body secondarily develops a fusiform shape in order to dimin- 

 ish resistance to the water in rapid swimming. 



Possible Causes of the Arrested Evolution of the 



Reptiles 



Of the eighteen great orders of reptiles which evolved on 

 land, in the sea, and in the air during the long Reptilian Era 

 of 12,000,000 years, only five orders survive to-day, namely, 

 the turtles (Testudinata), tuateras (Rhynchocephalia), lizards 

 (Lacertilia), snakes (Ophidia), and crocodiles (Crocodilia). 



The evolution of the members of these five surviving or- 

 ders has either been extremely slow or entirely arrested during 

 the 3,000,000 years which are generally assigned to Tertiary 

 time; we can distinguish only by relatively minor changes the 

 turtles and crocodiles of the base of the Tertiary from those 

 living to-day. In other words, during this period of 3,000,000 

 years the entire plant world, the invertebrate world, the fish, 

 the amphibian, and the reptilian worlds have all remained 

 as relatively balanced, static, unchanged or persistent types, 

 while the mammals, radiating 3,000,000 years ago from very 

 small and inconspicuous forms, have undergone a phenomenal 

 evolution, spreading into every geographic region formerly 

 occupied by the Reptilia and passing through multitudinously 

 varied phases not only of direct but of alternating and of 

 reversed evolution. During the same epoch the warm-blooded 

 birds were doubtless evolving, although there are relatively 

 few fossil records of this bird evolution. 



This is a most striking instance of the differences in chroma- 

 tin potentiality or the internal evolutionary impulses under- 

 lying all visible changes of function and of form. If we apply 

 our law of the actions, reactions, and interactions of the four 

 physicochemical energies (p. 21), there are four reasons why 



