51S F. H. EiyOWLTOy EVOLUTION OF GEOLOGIC CLIMATES 



Alaska entirely around the earth to eastern AnstraUa. or through more 

 than lob^ of latitude and more than 230" of longitude. Throughout 

 this vast, practieallv world-wide area, there is a remarkable uniformity 

 of distribution that is not only in individual species widely spread, but 

 considerable assemblages of species. Thus of the IT species known from 

 Cape Lisbume, Alaska, 8 are found in Amurland, eastern Siberia, 7 at 

 Irkutsk, 6 near Kamonka. Government of Elharkow, and 4: each in Mon- 

 golia and Caucasia and Turkestan, while with the Island of Bomhohn, 

 Sweden, there are 4 species in common, and i species occur in the 

 English beds, and S in Douglas County, Oregon. Of the 101 forms 

 found in the Jurassic of Oregon and California, 47 have an outside 

 range, there being 19 species common to eastern Siberia, 16 species at 

 Yorkshire, England, 5 species at Hope Bay, Graham Land, and smaller 

 numbers at widely scattered localities. Of the 23 species from Hope 

 Bay that are known elsewhere, 4 species occur in India, 9 at Yorkshire, 

 England, and, as already stated, 5 on our own Pacific coast. 



This distribution shows conclusively that there was not only free 

 communication between the Eastern and Western Hemispheres, but also 

 between the Yorthern and Southern Hemispheres, and as none of tli.i 

 Jurassic plants is known to possess any pecidiar mechanism for dis- 

 persal, it is apparently clear that there must have been a continuous 

 or practically continuous land connection throughout tliis vast area. 

 It is also evident that this wide distribution could only have been possibk 

 under very uniform climatic conditions. It remains to consider what 

 those climatic conditions probably were. 



The Jurassic flora that has been recovered to us numbers approxi- 

 mately 500 species, distributed among the following groups : Algales, 

 Marchantiales r, Filicales. Lycopodiales. Equisatales, Cycadales, Ben- 

 nettitales, Giokgoales, and Coniferales. 



The alga? and liverworts are so few in number and in general so obscure 

 and poorly preserved that little of value can be concluded regarding tho 

 environmental conditions under which they lived. The ferns are numer- 

 ous ia genera and species and usually abundant as individuals. A num- 

 ber of the fern genera have not yet been placed systematically, for the 

 reason that their essential organs have not been found, but, on the other 

 hand, there are a considerable proportion in which these features are 

 known, and hence these can be interrogate with a great degree of cer- 

 tainty as to the presumed climatic requirements. Thus we have Osmun- 

 dites. Todites. and Cladophlebis, that belong to the Osmundaceae, the 

 Hving members of which are largely tropical. ETuckia and Euffordia 



