8o EVOLUTION OF THE EARTH 



set up renewed evolution, resulting in the rise of new stocks. 

 In these various periodic physical changes we see the stimulus 

 that quickens organic evolution. Each cycle begins and closes 

 with a comparatively short time of accelerated evolution, while 

 the far longer intermediate tranquil periods lead to but little 

 that is striking in the way of new organisms. 



Conclusion 



From this summary of the earth's changing surface and 

 climate during geologic time the following facts may be 

 selected for emphasis : 



First, geologic time is very, very long, indeed so vast as to 

 be beyond human comprehension. No geologist today thinks 

 that the evolution of the earth and its life could have taken 

 place in less than lOO million years. My own view as a stu- 

 dent of Historical Geology is that geologic time endured about 

 800 million years. In all of this we perceive how slowly the 

 physical and organic processes bring about the results of nature. 

 Second, the constant shrinking of the earth leads to an in- 

 stability of surface that brings about periodic changes, not 

 only in the areal space relations of the water and land, but 

 in the shapes and heights of the lands as well. Third, no 

 sooner are the lands elevated above the sea-level than the 

 weathering processes become more active and through the 

 agency of the rain and wind all high places are, according to 

 human standards, slowly but surely moved into the seas and 

 oceans. Fourth, as a result of the transference of the high 

 lands into the water areas the latter are to a certain extent 

 displaced and periodically flood more or less of the lands. 

 Fifth, due to these surficial changes the atmosphere and the 

 climate are constantly changing in a small way, but every now 

 and then when the lands are largest, highest, and driest, a cold 

 period appears and disarranges the entire organic world, both 

 on the lands and in the waters. Sixth, when these "critical 



