A HISTORY OF SCIENCE 



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 mar- 

 supial order, which is the dominant mammalian type 

 of the living fauna of Australia to-day, existed in Eu- 

 rope and died out there in the tertiary age. Hence a 

 future geologist might think the Australia of to-day 

 contemporaneous with a period in Europe which in 

 reality antedated it by perhaps millions of years. 



All these puzzling features unite to render the sub- 

 ject of historical geology anything but the simple mat- 

 ter the fathers of the science esteemed it. No one 

 would now attempt to trace the exact sequence of 

 formation of all the mountains of the globe, as Elie de 

 Beaumont did a half -century ago. Even within the 

 limits of a single continent, the geologist must proceed 

 with much caution in attempting to chronicle the order 

 in which its various parts rose from the matrix of the 

 sea. The key to this story is found in the identifica- 

 tion of the strata that are the surface feature in each 

 territory. If Devonian rocks are at the surface in any 

 given region, for example, it would appear that this 

 region became a land surface in the Devonian age, or 

 just afterwards. But a moment's consideration shows 

 that there is an element of uncertainty about this, due 

 to the steady denudation that all land surfaces undergo. 

 The Devonian rocks may lie at the surface simply be- 

 cause the thousands of feet of carboniferous strata that 



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