3o6 EDMUND W. SINNOTT AND IRVING W. BAILEY 



indicating the occurrence of a rather mild winter. The advent of 

 the Glacial period resulted in very great extermination of the flora, 

 but herbs suffered much less than did woody plants. 



4. In America the vegetation was able to return northward after 

 the retreat of the ice and thus shows a considerable proportion of 

 woody species today. In northern Europe, however, natural 

 barriers have to a great extent prevented this return, and the present 

 flora of that region seems to be descended from the remnant of 

 the pre-Glacial vegetation which survived on the unglaciated areas. 

 The fact that the proportion of herbs in the present north European 

 flora is like that in northern or low alpine regions today provides 

 us with a clue as to climatic conditions during the height of the 

 glacial invasion. 



5. The land mass of the north temperate zone has been the 

 seat of origin of a very large part, if not of the majority, of herba- 

 ceous genera. The opportunities for wide dispersal and keen compe- 

 tition here have resulted in the production of a very hardy and 

 aggressive herbaceous flora which has spread widely into the tropics 

 and the Southern Hemisphere. 



6. This invasion of herbs from the north has taken place for the 

 most part during the Tertiary, and the percentage of herbs in the 

 floras of the various regions in the tropics and the Southern Hemi- 

 sphere indicates roughly the intimacy of the connection between 

 these regions and the land mass of the north temperate zone during 

 Tertiary time. The floras of certain oceanic islands, for example, 

 seem from the extreme paucity of herbs to be very ancient in type, 

 implying a very long period of isolation; whereas the floras of other 

 islands, overwhelmingly herbaceous, indicate a recent origin for 

 these islands or at least for their plant populations. 



7. Boreal herbs in the antipodes are usually separated from 

 their northern congeners by considerable distances, indicating that 

 migration has ceased and that the wide area of rather temperate 

 climate within the tropics, under which the invasion took place, 

 has recently become much smaller. 



8. A reconstruction and analysis of the ancient flora of the 

 antarctic continent indicate that up to comparatively recent times 

 its climate was very mild and that refrigeration took place more 

 rapidly than in the north temperate zone. 



