178 TREE ANCESTORS 



of the flowering plants. It was then this evolution during the Meso- 

 zoic that made possible the evolution of the mammals during the 

 Tertiary, so this far off happening of Mesozoic time might well be 

 considered as the primary stimulus to that chain of events that 

 resulted in humanity and humanity's civilization. 



The remains of tulip-trees were also a prominent element in the 

 forests that clothed the shores of the Upper Cretaceous sea that 

 about the same time spread northward from the Gulf of Mexico 

 up the Mississippi Valley and over the present prairie States. In 

 the shore sands of this advancing sea, which geologists call the 

 Dakota sandstone, a variety of tulip-tree leaves were preserved. 

 Some students have argued that since the leaves of the modem 

 tree are variable this variety of Cretaceous leaves may represent 

 a single variable species, but this is not beheved to have been the 

 case. 



There are at least 9 different kinds of tulip-tree leaves in the 

 Dakota sandstone ranging in appearance from the small Lirioden- 

 dron meekii (fig. 1) to the enormous obtusely-lobed Liriodendron 

 giganteum with leaves 7 inches in diameter, and the greatly pointed- 

 lobed Liriodendron quercifoliimi (fig. 2). Some of these forms have 

 been found from Texas to Iowa, Kansas and Nebraska, and 7 

 of them are confined to the Dakota sandstone. A similar display 

 has been found in the Upper Cretaceous deposits of the Atlantic 

 Coastal Plain from Long Island to Alabama and Arkansas. There 

 is 1 recorded from Long Island, 3 from New Jersey, 2 from North 

 Carolina, 1 from Alabama, and 2 from Arkansas. One of these 

 known as Liriodendron meekii is found in western Greenland, Ne- 

 braska, in Saxony, and it has even been recorded from Argentina, 

 although this last record is not properly authenticated. 



At the time that these tuhp-trees were flourishing in Greenland, 

 southeastern North America and in the western interior, other 

 forms were present on our Pacific coast, two different forms having 

 been discovered in the Upper Cretaceous rocks of Vancouver Island. 

 Somewhat later in Cretaceous time a second species occurred in 

 Saxony, and toward the close of the Cretaceous two additional 

 American species appear in the record — one in western Tennessee 



