306 FLOKA OF LOWER COAL MEASURES OF MISSOURI. 



individual species, and this in turn is much less impressive than the uni- 

 formity of the sequence and the parallelism of their appearance and 

 extinction during- this epoch, or the similarity of the elements which com- 

 posed each flora. The writer is disposed to believe that the conditions 

 favorable for plant distribution and the consequent comparatively homo- 

 geneous dispersion of the successive floras of the northern hemisphere 

 during the period extending' from the later Culm to near the middle of the 

 Mesocarboniferous have never been equaled since. That there was plant 

 migration can not for a moment be questioned. Yet the evidence of dis- 

 tribution, of A'ertical range, of characteristic associations, and of the suc- 

 cession of the floras bespeaks for the terrestrial plant species of that period 

 such geographic iiniformity of climate and such facility of intermigration, 

 probably over a minimum distance, as to justify us in regarding the aston- 

 ishingly similar associations of identical or closeh' related g-enera and 

 species which characterize each stage, zone, or group of the Culm and 

 Mesocarboniferous as essentially contemporaneous in all the basins of the 

 northern hemisphere. 



Whether the Carboniferous flora was developed within the arctic zone 

 or some other region of the earth is hardly more than a subject for specu- 

 lation. Personally I am inclined to believe that many of the species or 

 genera of the Mesocarboniferous were, under similar local conditions, 

 evolved in different portions of the laud surface, whence they spread, with 

 a rapidity diflicult to conceive in the present day, over the greater part of 

 the northern continents. Such a mode of generation, at different points, 

 of the various elements comprising a given flora might be described as 

 polychthanous. 



The suggestion ofl^ered at different times by several European paleo- 

 botauists^ that the flora of Mazon Creek, which is generally cited in America 

 as the familiar illustration of the plant life of the lower part of our Lower 

 Productive Coal Measures, really represents a stage much higher than the 

 lowest series above the Millstone grit of Europe seems to be fully corrobo- 

 rated by a comparative examination of the floras. Such an examination 

 will show, if we accept the synchronology of the respective floras, that the 

 plants of the Middle Kittanning, or of the E coal, fall within and are 

 apparently nearly contemporaneous with the Geistlautern beds or the upper- 



' See Zeiller: Fl. foss. bassin houill. Valeucienaes, p. 195. 



