458 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



the vegetation of the world as that now held by the oaks and other 

 familiar forest trees included in the oak family. Omitting any further 

 reference to the Paleozoic fossils, mentioned on a previous page as 

 possible though doubtful members of the Ginkgoalean stock, the main 

 historical conclusions may be summarized as follows (see fig. 2). 



As the Triassic period drew to its close more than 150 million years 

 ago, the surface of the earth bore little or no resemblance to that with 

 which we are familiar. At that stage in geological history and during 

 many million years that followed, the distribution of land and water, 

 mountains, valleys, and plains, animal and plant communities, and 

 climatic conditions reflected in their gradual transformation the cycles 

 of physical and organic evolution. Even in the Triassic period there 

 were a few plants foreshadowing more or less clearly trees that are 



Figure 1.— Map of the distribution of Ginkgo and allied genera in former periods of geological history. 

 The area within which Ginkgo may still exist as a wild tree is shown by the circle below 30° N. lat. on 

 the eastern edge of China. The records on which the map is based are from the Triassic to the Quaternary 

 period. 



still living. Conspicuous in this small company were representatives 

 of the Ginkgoalean family, notably Baiera and Ginkgo. Passing up- 

 ward to the Jurassic period we find other genera added to the growing 

 family, genera which established themselves over a vast extent of 

 territory both north and south of the Equator. It was at this stage 

 in the history of the earth that Ginkgo, Baiera, Phoenicopsis, Czeka- 

 nowskia and other genera reached their greatest development as 

 measured by the number of species and geographical range. As the 

 Jurassic period merged into the Cretaceous, the balance of nature was 

 not seriously disturbed: many genera survived the change. But the 

 Ginkgoalean race had passed its zenith. There followed a much more 

 drastic physical revolution when the Cretaceous sea, in which the 

 material of our chalk downs was made from the calcareous skeletons of 

 marine creatures, flooded vast continental areas — a revolution which 

 had a far-reaching effect upon contemporary life. 



