THE (iEOCiRArillCAL DISTRIBUTION OF I'LAXTS. Ill 



In fact, the cases in which man has produced any considerable 

 effect upon the vegetation or flora of a region are extremely few, 

 and the effect is nearly always destructive. He often clears aAvay 

 an entire forest leaving in its place hideous, naked barrenness, but 

 where has he made a wilderness to blossom as the rose, — with 

 wild plants which could sustain themselves after his watchful care 

 is removed? The Island of St. Helena is a case in point, in 

 which by the combined action of the axe and of introduced 

 browsing animals, principally goats, a rich vegetation, peculiar to 

 that island, has been nearly exterminated from off the face of the 

 earth. 



And now we approach that part of our subject which I am sure 

 wiU prove the most interesting to you, — the distribution of the 

 great floras of the earth. A flora is simply a great group of 

 plants which have had a common history. There is no fact better 

 established in geology than that -the earth's crust is not stable but 

 is constantly undergoing extensive elevations and subsidences ; 

 this brings it to pass that land and water surfaces are not constant 

 but changing, and that many regions now separated by the sea 

 have in former times been connected, and many now connected 

 have but lately become so. These changes have forced vegetation 

 to perform migrations which have been small or extensive, slow or 

 rapid, just in proportion to the change, and these have been 

 accompanied by great changes in climate, including even such 

 extreme conditions as prevailed in the glacial period. 



In looking at the Distribution of Plants in a broad way, the 

 first feature to strike attention is the constancy with which upon 

 higher mountains we find plants characteristic of regions far to the 

 north of them in the northern hemisphere, and far to the south of 

 them in the southern — and in all cases cut off from their brethren 

 by many valleys and plains occupied by totally distinct species. 

 Thus, upon the Rocky Mountains, the Alps, the Himalayas, and 

 others, are little herbaceous plants which are found growing at the 

 sea level around the Arctic ocean. Farther down these mountains 

 come plants which grow in more southern parts of Alaska, 

 Labrador, northern Scandinavia, and Siberia. Now it is plain 

 why they can live there — the conditions of heat and moisture 

 upon the mountain side and at the sea level farther north, are 

 similar, as we have seen ; but how did the plants get upon the 

 mountains from the North? The natural means of spread of 



