102 Dynamic Theory. 



world. But he was not exterminated from the earth, for the Opossum 

 existed in i ^Europe in Miocene times, and he exists at this day in the 

 United States 1 besides, n'idtfiy Relative marsupials in other countries. 



At jEhefcenS pf th.e* Cretaceous .period the table estimates a space of 15,- 

 00 0, 00 O'yea'rs titling ^JcliUhe .'European strata were repeatedly tilted 

 up and pared off, and the mountain ranges in western North America 

 were shoved up. The northern regions of both continents were sub- 

 jected to repeated oscillations keeping them both pretty well elevated 

 much of the time and at last leaving them above the sea as now. 



In the Rocky Mountain region in western United States the Cretaceous 

 beds are now from 4,500 to 7,000 feet above the ocean. This eleva- 

 tion has all taken place since the beds were formed under the sea and 

 the most of it before the fresh water deposits of the Eocene could take 

 place. So that although in America there is not the unconformability 

 of strata that exists in Europe, yet the time which makes up the inter- 

 regnum between the Mesozoic and Tertiary may have been equally long 

 while more orderly. Yet it is not improbable that a part of the Creta- 

 ceous layers of America were put down during the disturbances in 

 Europe and that the real halt between the periods of rock building was 

 less here than there. 



But during this long period from the end of the Jurassic to the be- 

 ginning of the Eocene, which the table estimates at 24,000,000 years, in 

 some primitive continent which might perhaps have occupied the site of 

 the Indian ocean and South Pacific, with connection to Asia and South 

 America and from North Asia to North America, mammal types had 

 developed from the marsupials, and as the Asiatic, European and Amer- 

 ican continents became suited to their needs they spread themselves over 

 them. America was probably invaded from the southwest or north- 

 west, and Europe from the southeast. The species in the two conti- 

 nents in Tertiary times are nearly enough related to indicate a common 

 origin, but not near enough to show that one was necessarily derived 

 from the other. As shown above in the cases of the horse and camel, 

 the first that came to America were already differentiated in an impor- 

 tant manner from their original marsupial ancestors ; a differentiation 

 that certainly took place before the families diverged from each other 

 in the primitive continent. After reaching America the modifications 

 forced upon them here are easily visible running through say 15,000,- 

 000 years as estimated by the table. It is indeed not impossible that 

 during much of this time there was land all the way from western 

 Europe eastwardly across Asia and the North Pacific to North America, 

 But even if a single species were distributed across this whole belt, it 

 would not be long before the varying conditions of climate and food 

 along this belt would modify the various sections of the species into new 



