E. V. Wulfif 



-190- 



Historical Plant Geography 



sperms of that period, cannot long remain submerged in sea water 

 without injuring the embryo, so that transport by sea currents is im- 

 probable. Of birds at that time there existed only Archaeopteryx, and 

 mammals had only just begun to make their appearance. There was 

 also no possibility of wind transport, since Ginkgo has very large, 

 heavy seeds without any adaptations for flight. This connection of 

 Greenland and North America with Eurasia was broken, according to 

 Wegener, only in the Quaternary period. This genus extended its 

 range eastward and westward from the indicated Angara center in 

 strict conformity with the location of the 40th parallel. The absence 

 at this time (at the close of the Jurassic and the beginning of the 

 Cretaceous period) of any considerable shiftings of area in the merid- 

 ional direction is in accord with the supposition of Koppen and 

 Wegener that climate changed very little during these geological 

 periods. 



^Ae C/'e^oceous ar-ea. 



on/y o^ ^Ae e/tty ©y ;f/>s Cye^crceous. 



Fig. 32. — Areas of Ginkgo adiantoides Unger in different geologic periods. The 

 geographic network (0°, 30°, 60°) and the shapes of the mainlands are those of the Cretace- 

 ous. (After Shapaeenko). 



At the beginning of the Tertiary period there took place an ex- 

 tension of the area of Ginkgo in North America toward the north, 

 which corresponds to the change to a warmer cHmate that occurred 

 here and reached its maximum during the Eocene stage. In the 

 Oligocene and Miocene stages the climate over the entire Eurasian 

 continent and also over North America began to get colder. This 

 caused a shifting of the area of Ginkgo to the south. Prior to the 

 Pliocene stage it had already reached as far south in North America 

 as British Columbia and in Europe as Sterhtamak (Bashkiria). During 

 the Pliocene stage Ginkgo became extinct in America, and in Europe 

 there remained only a small, isolated area in southern France, which 

 was completely wiped away by the Quaternary glaciation. Ginkgo 

 has thus been preserved to the present day only in eastern Asia, a 

 region which was not subjected to glaciation and which has suffered 

 least of all from great climatic changes. This accounts for the preser- 



