PALEOBOTANICAL EVIDENCE 509 



least two groups, one of which (the Cordaitales) probably formed forests, 

 the seed-bearing habit was clearly established, though both failed to sur- 

 vive the Paleozoic. 



The place of origin of this flora was apparently in eastern Nortii 

 America or the adjacent Arctic regions. It was, however, much more 

 widely distributed, many of the same, or closely related forms, ranging 

 from Pennsylvania and New York through eastern Canada, Ellesmere- 

 land, Spitzbergen, Bear Island, in the Arctic Ocean, the Don region of 

 Kussia, the British Islands, Belgium, and even reaching Queensland and 

 Victoria. There is a noteworthy bed of coal of this age, 3% feet in 

 thickness, on Bear Island. 



The interpretation of the climatic conditions under which the Devo- 

 nian floras existed is beset with difficulties. So little is definitely known 

 concerning the flora of the early Devonian that not much can be said oi* 

 the probable climatic requirements, though it is difficult to understand 

 how this flora, so newly emerged from the water and composed of thin, 

 delicate-leaved types, could have endured except under uniform and 

 moist conditions, and it seems a safe inference that the temperature must 

 at least have been warm. 



The climatic requirements of the Middle Devonian flora are somewhat 

 difficult to interpret. Strange and harsh in aspect as this flora is, it 

 has left little in linear descent and hence it is almost impossible to know 

 the precise conditions under which it could have lived. The fact that 

 it is so widely distributed over the Northern Hemisphere implies that 

 the conditions were uniform and without obvious climatic zones. Its 

 origin in or free migration over Arctic lands points at least to mild 

 conditions. This view is supported by evidence of marine invertebrates. 

 Schuchert says: ^*l\^armer conditions again prevailed in the Northern 

 Hemisphere early in Middle Devonic times, for coral reefs, limestones, 

 and a highly varied marine life with pteropod accumulations were of 

 wide distribution." 



The Upper Devonian floras, however, are more easily and safely inter- 

 preted. Their wide distribution, which includes not only the Eastern 

 and Western, but also the Northern and Southern Hemispheres, shows 

 that the conditions of uniformity were still maintained, while the com- 

 plete absence of growth rings in the woods shows the absence of severe 

 changes in temperature or intervals of prolonged drought. Therefore 

 the climate must have been equable and at least mild. 



There is undoubted evidence of local and more or less circumscribed 

 glaciation in the later stages of Devonian time, as, for example, in New 



