PLANT EECOEDS OF THE ROCKS — SEWARD 369 



furnish striking examples of persistence through the ages of certain 

 plants that are relatively small and simple. 



The study of extinct floras shows very clearly that while group 

 after group were evolved, increased in numbers, spread over a large 

 part of the world and then rapidly declined or died out, other prod- 

 ucts of evolution held their own with little or no essential change 

 from the remote past to the present day. 



At the close of the Carboniferous period the face of the earth 

 changed ; foldings of the crust brought into being mountain ranges ; 

 forests were replaced by the sparse vegetation of semiarid regions. 

 This geological revolution had its repercussion in the course of 

 organic evolution; the changed environment, which followed as the 

 natural consequence of crustal changes, was a potent factor in alter- 

 ing the composition and the nature of the world's floras. Failure to 

 discover undoubted pteridosperms in rocks formed subsequently to 

 the Paleozoic era led to the belief that these seed-bearing fernlike 

 plants failed to retain a hold on life in the altered circumstances 

 caused by the revolution in the inorganic world. Evidence has re- 

 cently been brought forward by Dr. Hamshaw Thomas and Dr. T. M. 

 Harris, of Cambridge, definitely showing that some pteridosperms 

 continued to play an important part in the vegetation of the earlier 

 stages of the Mesozoic era. It was suggested by the writer several 

 years ago that certain fernlike fronds that are abundant in Triassic 

 and Jurassic floras might belong to pteridosperms; this opinion 

 has now received some confirmation through the discovery of both 

 male and female organs — pollensacs with pollen, and seeds — which 

 undoubtedly belong to genera, e. g., Lepidopterls, which most 

 paleobotanists had classed among the ferns. 



Results obtained by students of fossil plants raise many problems 

 other than those more directly connected with the great problem of 

 evolution. We are accustomed to associate assemblages of living 

 plants with different climatic conditions, and it has been said that 

 the application of knowledge gained from the present enables us to 

 use fossil plants as " thermometers of the ages." One example will 

 serve to illustrate this aspect of paleobotany. The greater part of 

 the island of Greenland is now buried deep under vast ice sheets; 

 it is only on the narrow coastal fringe and on a few isolated peaks 

 (Nunataks) protruding through the ice that plants are able to com- 

 plete their life cycle in the extremely short Arctic summer. Except 

 in the district near Cape Farewell there are no trees — only willows 

 not more than about 3 feet high and a prostrate birch. The flora, 

 excluding the lower plants, includes nearly 400 species of flowering 

 plants, a few ferns and their allies; it is a typical Arctic flora with 

 temperate associates. On the west coast of Greenland and on Disko 



