THE STORY OF NINETEENTH-CENTURY SCIENCE 



years earlier or later than the period whose records are 

 said to represent the Devonian age in America. In at- 

 tempting to decide such details as this, mineralogical 

 data fail us utterly. Even in rocks of adjoining regions 

 identity of structure is no proof of contemporaneous 

 origin ; for the veritable substance of the rock of one 

 age is ground up to build the rocks of subsequent ages. 

 Furthermore, in seas where conditions change but little 

 the same form of rock may be made age after age. It 

 is believed that chalk beds still forming in some of our 

 present seas may form one continuous mass dating back 

 to earliest geologic ages. On the other hand, rocks dif- 

 ferent in character may be formed at the same time in 

 regions not far apart say a sandstone along shore, a 

 coral limestone farther seaward, and a chalk bed be- 

 yond. This continuous stratum, broken in the process 

 of upheaval, might seem the record of three different 

 epochs. 



Paleontology, of course, supplies far better chrono- 

 logical tests, but even these have their limitations. 

 There has been no time since rocks now in existence 

 were formed, if ever, when the earth had a uniform 

 climate and a single undiversified fauna over its entire 

 land surface, as the early paleontologists supposed. 

 Speaking broadly, the same general stages have attend- 

 ed the evolution of organic forms everywhere, but there 

 is nothing to show that equal periods of time witnessed 

 corresponding changes in diverse regions, but quite the 

 contrary. To cite but a single illustration, the marsupial 

 order, which is the dominant mammalian type of the 

 living fauna of Australia to-day, existed in Europe and 

 died out there in the Tertiary age. Hence a future 

 geologist might think the Australia of to-day contempo- 



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