The Wallace-Darwin Correspondence \ 



true glacial period possible with high excentricity. When 

 the high excentricity passed away the glacial epoch also 

 passed away in the Temperate zone; but it persists in the 

 Arctic zone, where during the Miocene there were mild 

 climates, and this is due to the persistence of the changed 

 geographical conditions. The present Arctic climate is 

 itself a comparatively new and abnormal state of things 

 due to geographical modification. As to *^ epoch " and 

 ^* period," I use them as synonyms to avoid repeating the 

 same word. 



3. Rate of deposit and geological time : there no doubt 

 I may have gone to an extreme, but my ^' twenty-eight mil- 

 lion years '^ may be anything under 100 millions, as I state. 

 There is an enormous difference between 7nean and maximum 

 denudation and deposition. In the case of the great faults 

 the upheaval along a given line would itself facilitate the 

 denudation (whether subaerial or marine) of the upheaved 

 portion at a rate perhaps a hundred times faster than plains 

 and plateaux. So, local subsidence might itself lead to very 

 rapid deposition. Suppose a portion of the Gulf of Mexico 

 near the mouth of the Mississippi were to subside for a few 

 thousand years, it might receive the greater part of the sedi- 

 ment from the whole Mississippi valley, and thus form strata 

 at a very rapid rate. 



4. You quote the Pampas thistles, etc., against my 

 statement of the importance of preoccupation. But I am 

 referring especially to St. Helena, and to plants naturally 

 introduced from the adjacent continents. Surely, if a cer- 

 tain number of African plants reached the island and became 

 modified into a complete adaptation to its climatic condi- 

 tions, they would hardly be expelled by other African plants 

 arriving subsequently. They might be so conceivably, but 

 it does not seem probable. The cases of the Pampas, 

 New Zealand, Tahiti, etc., are very different, where highly 



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