104 Dynamic Theory. 



be there too, of course. If these records could be reached it might ap- 

 pear that the larger mammals existed in America before the Eocene. 

 At any rate we have no more reason to doubt that they existed on some 

 other continent before the Eocene than we have to doubt that the mar- 

 supials were living somewhere else than America and Europe during the 

 Cretaceous. 



Le Conte suggests that there may be periods of extraordinary devel- 

 opment; when changes in the environment of organic life are rapid 

 enough to impose great and rapid differentiations on the organisms, but 

 not so rapid as to kill them. No doubt there are such periods, but we 

 must remember that the most rapid development is an exceedingly slow 

 process. It is improbable that the mammals found in the Eocene of 

 America could have been, developed during that period from any animals 

 here before that time. But the theory of emigration from a Primitive 

 Continent gives all the time from the end of the Carboniferous to the 

 beginning of the Tertiary for the development on that continent of the 

 mammals from their oviparous ancestor by way of ovoviviparous mono- 

 tremes and marsupials a period estimated by the table at 55,000,000 

 years. 



On the whole it appears that the geological record as we have it so 

 far, is a largely mutilated volume. Alternate chapters have been torn 

 out as it were, and while we easily catch the general drift of the history, 

 details here and there have escaped us. 



Another important consideration regarding geological history is still 

 to be mentioned ; and that is, that geological times of the same name in 

 different parts of the world are not necessarily synchronous. The only 

 way to identify strata laid down in remotely ancient times in different 

 regions is by the identity or equivalence of their organic remains. If 

 the theory that organic beings have peopled the different parts largely 

 by successive waves of emigration from an original center, or a shifting 

 succession of centers ; it would naturally follow that the same families 

 and genera would arrive at different localities in successive periods of 

 time. There would be modifications in the character of the species dur- 

 ing the migration, so that when installed in a new land some ages after 

 leaving an ancient hive, the original family is represented by a new 

 species, but because the family characteristics are retained, the age in 

 the two cases is regarded geologically as the same. 



