3i8 NOTES OF A NATURALIST. 



ocean during the long course of geological history, 

 and take into account that the same great mountain 

 region also furnished materials to streams which 

 flowed northward and eastward. 



In attempting to speculate on the past history of 

 this region it is important to remark that, so far as 

 evidence is available, there is reason to believe that 

 Brazil has undergone less considerable changes of 

 level than most other parts of the earth's surface. 

 Even if we go back to the period of the earlier 

 secondary rocks, there is no evidence to show that 

 movements of elevation or depression have exceeded 

 a few hundred feet. 



I have attempted elsewhere * to give a sketch of 

 the views which I hold as to the probable origin of 

 the chief types of phanerogamous vegetation. I there 

 pointed out that, at a period when physical conditions 

 in the lower regions of the earth's surface were widely 

 different, and the proportion of carbonic acid gas 

 present in the atmosphere was very much greater 

 than it has been since the deposition of the coal 

 measures, it was only in the higher region of great 

 mountain countries that conditions prevailed at all 

 similar to those now existing. I further argued that, 

 if the early types of flowering plants were confined, 

 as I believe they were, to the high mountains, we 

 could not expect to find their remains in deposits 

 formed in shallow lakes and estuaries until after the 

 probably long period during which they were gradu- 

 ally modified to adapt them to altered physical con'- 

 ditions. 



* Proceedings of the Royal Geographical Society for 1879, p. 564. 



