1867-1882] ISLAND LIFE 23 



As to " epoch " and " period," I use them as synonyms to Letter 396 

 avoid repeating the same word. 



3. Rate of deposition and geological time. Here no doubt 

 I may have gone to an extreme, but my " 28 million years ' 

 may be anything under 100 millions, as I state. There is an 

 enormous difference between mean and maximum denudation 

 and deposition. In the case of the great faults the upheaval 

 along a given line would itself facilitate the denudation 

 (whether sub-aerial or marine) of the upheaved portion at a 

 rate perhaps a hundred times above the average, just as 

 valleys have been denuded 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 mouths of the Mississippi, were to subside 

 for a few thousand years, it might receive the greater portion 

 of the sediment from the whole Mississippi valley, and thus 

 form strata at a very rapid rate. 



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

 ment 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 certain number of 

 African plants reached the island, and became modified into 

 a complete adaptation to its climatic conditions, they would 

 hardly be expelled by other African plants arriving subse- 

 quently. 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 developed aggressive plants 

 have been artificially introduced. Under nature it is these 

 very aggressive species that would first reach any island in 

 their vicinity, and, being adapted to the island and colonising 

 it thoroughly, would then hold their own against other plants 

 from the same country, mostly less aggressive in character. 



I have not explained this so fully as I should have done 

 in the book. Your criticism is therefore useful. 



5. My Chapter XXI II. is no doubt very speculative, and 

 I cannot wonder at your hesitating at accepting my views. 

 To me, however, your theory of hosts of existing species 

 migrating over the tropical lowlands from the N. temperate 

 to the S. temperate zone appears more speculative and more 

 improbable. For where could the rich lowland equatorial 

 flora have existed during a period of general refrigeration 



