The Problems of Plant Distribution 239 



prototypes. These forms, although comparatively 

 few in number at the present time, are of remarka- 

 bly wide distribution, many of the genera being 

 cosmopolitan. The distribution of the liverworts is 

 especially interesting in this connection, and in most 

 cases can be explained only on the hypothesis that 

 they are survivors of widely distributed types, which 

 have come down probably from the Paleozoic with 

 little change. This is especially the case in such 

 tropical genera as Dumortiera and Monoclea. 



Fossil Pteridophytes. The history of the Pteri- 

 dophytes is much easier to trace, as there are 

 abundant fossil remains, evidently closely related to 

 many living types, and indeed some living genera 

 can probably be traced back to the Paleozoic. The 

 Paleozoic ferns are for the most part of the so- 

 called " eusporangiate " type, and are allied to the 

 living ferns of the family Marattiaceae, which at 

 present are found mainly in the tropics. While 

 many of the " ferns " of the Paleozoic are now 

 known to be seed-bearing plants, there is no rea- 

 sonable doubt that true ferns, allied to the Marat- 

 tiaceae, were abundant in the Paleozoic flora. 



Climatic Changes in the Permian. The last 

 period of the Paleozoic, the Permian, was an era of 

 transition. There are evidences of severe glaciation 

 in the Southern Hemisphere, and less marked glacia- 

 tion in many regions in the Northern Hemisphere, 

 where in a good many places it is also evident that 

 marked aridity prevailed, in strong contrast to the 



