134 NEW YORK STATE MUSEUM 



dependent on the exterior agencies supplied by the ever changing 

 physical conditions, which stimulate the four inseparable factors 

 of evolution. As seen from the side of the persistent forms, vari- 

 ability and development in the organic world stop when the physical 

 conditions become stationary. 



This conclusion from the persistence of types is corroborated by 

 evidence from the opposite phenomenon, that there are periods of 

 rapid development under the stress of severe and rapidly changing 

 physical conditions, such as, for instance, obtained in the periods 

 of maximum emergencies of the continents that separated the 

 geologic eras. 



While we stand in awe before the orderly and apparently wilful 

 steady development of the organic world to its highest expression, 

 man, the persistent types remind us what might have happened if 

 the surface of the earth had remained unchanged and unchange- 

 able, if the advancing and receding seas, rising and sinking con- 

 tinents and mountain ranges, drying up seas, rivers and lakes, 

 changing climates and other physical factors had not continu- 

 ously subjected the organic world to new conditions and new 

 stresses, ever propelling them forward, letting the oceanic organ- 

 isms into the epicontinental seas and forcing them back again into 

 the oceans, or driving them into the rivers and thence upon the 

 dry land, in successive waves of worms, insects, arachnids and 

 vertebrates ; and from the dry land even into the air. There is no 

 doubt that without these driving forces a stagnant, lowly organ- 

 ized world of relatively persistent types would have resulted. 



