PLANT RECORDS OF THE ROCKS — SEWAED 371 



only an assumption, that Wegener's hypothesis of continental drift 

 expresses a truth — in other words, the land that is now Greenland 

 may have been farther from the North Pole than it is now ; second, 

 as already suggested, it is arguable that the relation between living 

 plants and climatic conditions should not be accepted as applicable 

 to the plants of other genera and species which lived about a hundred 

 million years ago. The Cretaceous flora of Greenland is one of many 

 ancient floras which raise puzzling and fascinating problems by no 

 means easy of solution. 



Reference has been made to evidence furnished by fossil plants of 

 the gradual rise to prominence in the Paleozoic era of groups of 

 plants which after a time of vigorous development became almost or 

 completely extinct; evolution of the plant kingdom was character- 

 ized by the rise and fall of dynasties that are no longer with us. The 

 study of fossil plants also affords striking instances of the remote 

 antiquity of some genera that are still in being. The maidenhair 

 tree that is often seen in cultivation is the solitary survivor of a 

 group which in all probability traces back its ancestry to the latter 

 part of the Paleozoic era. In the early stages of the Mesozoic era, 

 particularly in the latter part of the Triassic period and through the 

 whole of the Jurassic period, the ginkgo group was represented by 

 many different genera, all of which^ with the exception of Ginkgo^ 

 have long been extinct. Ginkgo Mloha was aptly styled by Darwin 

 " a living fossil " ; it is unquestionably one of the most ancient types 

 in the world. It was once almost cosmopolitan in its geographical 

 range and now it lives only where man has planted it. 



A retrospect through the ages shows that there has been an evolu- 

 tion, an unfolding of innumerable structural forms, some more com- 

 plex and larger than their nearest living relatives, destined to endure 

 for a time, then to disappear. Evolution was not a simple uniform 

 progression as was formerly believed. We are still unable con- 

 fidently to picture the procession of classes, groups, and families 

 through the hundreds of millions of years that have elapsed since 

 the earth received on its surface the pioneers of the plant kingdom. 

 Imperfect as the geologic record certainly is, we can hopefully look 

 forward to learning more of the mysteries of evolution from the 

 records of the rocks than from any other source. The records, though 

 lamentably incomplete, are rich in treasures worthy of study. The 

 unexplored material both in museums and in the rocks is enormous; 

 the workers in this fruitful field are unfortunately very few, 



149571—33 25 



