26 ME. J. BALL ON THE FLOEA 



gradual modification, I see no difficulty in believing that they may 

 have maintained themselves through successive gradual changes 

 of physical conditions within the same regionj and even that some 

 may still survive within the Antarctic Circle. This conjecture pre- 

 sented itself forcibly to my mind when passing some days in mid- 

 winter at Sandy Point in the Straits of Magellan. Many delicate 

 plants, impatient of sharp frosts, maintain themselves there in 

 the same latitude with not distant places, such as South Georgia, 

 which are almost completely covered with permanent snow. The 

 reason is sufficiently obvious. The accumulation of snow or ice 

 taking a permaneut form occurs only where the supply, by pre- 

 cipitation or condensation, exceeds the amount annually melted, 

 and this is limited to special portions of the earth's surface, 

 usually to high lands, exposed to moist currents from the sea 



Having considered the constituents of the Andean flora which 

 have most probably been derived from other regions of the earth, 

 I may briefly notice the remaining types which are peculiar to 

 this part of the earth. Erom the figures given in the preceding 

 pages we are led to conclude that the large majority of the species 

 and about one half of the genera of the Andean flora are not 

 known elsewhere. Of these endemic species and genera the 

 majority are allied to the cosmopolitan types to which I have so 

 often referred, and extend northward to Central America and 

 Mexico. Others, and especially the genera which exhibit less 

 affinity to types known elsewhere, are confined to the proper 

 range of the South-American Audes, and sotne of them to a small 



# 



portion of the range. We find in these facts reason to believe 

 that, while South America has from a remote period been able to 

 receive occasional colonists from the north, there has been a very 

 long period of relative isolation during which a large number of 

 separate species, and not a few genera, have been differentiated. 

 As to some groups at least, such as the Loasacecey many Mutisi- 

 acece, and others where the structure has been profoundly modified, 

 we must allow a very long interval for changes so considerable. 

 If it be objected that we have reason to believe the elevation of 

 the Andean chain to be a comparatively recent geological event, 

 I would reply, in the first place, that there is no reason to suppose 



* This is not the place to discuss the views which Dr. James CroU has main- 

 tained with singular ability and a wide grasp of the facts and principles of 

 physical science. I find it impossible to accept his theory of a polar ice-cap and 

 the important consequences which he derives from it. 



