THE MAIDENHAIR TREE SEWARD 



459 



When the floor of the Cretaceous sea had been raised into land 

 and the calcareous ooze of the ocean was converted into hills of 

 chalk, the plant world assumed a much more modern aspect. In 

 the Tertiary floras preserved in sedimentary deposits of post Cre- 

 taceous age Ginkgo was almost, though not quite, the sole survivor 

 or the family; it was still a vigorous and widely dispersed tree in 



R 



T Q-R 



Bakra 



Qwbgodium 



Ewtmophyllum 



Tovzllia \ 

 PszudotoveRia J 

 Phoenicopsis 

 Czekmowskia 



Hartzia, 



Cul&weria 



Skphenophyllum 



Windwardia 



Sphenobaiem 



Arctobakra 



Figure 2.— Diagram illustrating the approximate distribution in time of genera of the Ginkgoales. 

 P— Q— R = Perrnian, Triassic, Rhetie, Jurassic, Cretaceous, Tertiary, Quaternary and Recent periods. 

 The relative antiquity of the Ginkgoales and the human race is indicated by the black time scale. Accord- 

 ing to the late Prof. Elliot Smith (Human History) man "must have been alive during the Pliocene stage." 

 Prof. A. Holmes (The Age of the Earth) gives the date of the Triassic period as 180 million years. The 

 name Ginkgo is used in a wide sense, including some species which should be assigned to Ginkgoites. It 

 is important to note that the genera Stephenophyllum and others shown in the Table as Lower Cretaceous 

 were founded on exceptionally well-preserved fossils from Franz Josef Land; in all probability, were 

 equally good material available from other places, they would be found to have had a much greater range 

 in time and in space. The diagram is not drawn to scale except that the Rhetie period, transitional 

 between the Triassic and Jurassic, is shown as a shorter phase of geological history. 



American, Arctic, European, and Asiatic forests. Even as late as the 

 last, or Pliocene, stage of the Tertiary period Ginkgo still lingered on in 

 Europe. It is probable that the genus was unable to endure the severe 

 arctic conditions which swept over an enormous area in the Old and the 

 New World when the genial climate of the Tertiary age was followed 

 by a glacial phase. Ginkgo lived on in the Quaternary period, the 



