THE GEOLOGICAL HISTORY OF GYMNOSPERMS* 



EDWARD WILDER BERRY 

 The Johns Hopkins University, Baltimore, Md. 



There are two considerations that I desire to emphasize be- 

 fore attempting to sketch the geological history of gymnosperms. 

 These are (1) the vast period of earth history that had elapsed 

 before any recognizable traces of vascular plants are found in 

 the record, and (2) the reasons why they have not yet been found 

 in the earlier Paleozoic periods. One is apt to speak of the 

 Paleozoic flora as if it were an entity comparable to the recent 

 flora although the Permian gymnosperms were separated from 

 their known Devonian ancestors by a far greater interval than 

 separates the existing flora from pre-glacial floras. A chronology 

 is then one of the most important factors in any understanding 

 of the succession of floras that have clothed the earth. 



There are a great many ways of estimating geological time, 

 most of them dependent on the- comparisons of past processes 

 with present processes, and all nothing more than approxima- 

 tions. It is possible, however, to get an idea of the relative dura- 

 tions of the different periods that will be reasonably accurate. 

 Geologists segregate the rocks of the earth's crust, into five sys- 

 tems — Archeozoic, Proterozoic, Paleozoic, Mesozoic and Ceno- 

 zoic. If 5 be taken to represent the duration of the Cenozoic 

 then the Mesozoic will be represented by 12, the Paleozoic by 

 27, the Proterozoic by 27 and the Archeozoic by 30. Some 

 idea of the vastness that these figures represent may be gained 

 from the statement that the Swiss Alps, Himala3'as, Pyrenees, 

 and Rockies have been elevated and the Colorado Canon eroded 

 since the dawn of the Cenozoic ; and neither mammals nor angio- 

 sperms are found as far back as the beginning of the Mesozoic. 



* Paper read in the Symposium on Gymnosperms at the meeting of the 

 Botanical Society of America at the University of California, August 3, 1915. 



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THE PLANT WORLD, VOL. 19, NO. 2, 1916 



