72 TREE ANCESTORS 



nuts and husks in the buried swamp deposits. The following still 

 existing species have been recorded from the Pleistocene of this 

 country : the White Hickory {Hicoria alba) from a cave in Pennsyl- 

 vania and from the interglacial beds near Toronto, Canada; the 

 Water Hickory {Hicoria aquatica) from North Carolina; the shag 

 bark {Hicoria ovata) from Pennsylvania, Maryland and North 

 Carolina; and Hicoria glabra from Pennsylvania, Maryland, Vir- 

 ginia, and North Carolina. 



The accompanying map (fig. 12) shows the area occupied by 

 the existing hickories in sohd black, and the area within which 

 Tertiary fossil forms have been found by vertical lining. It seems 

 probable that extinct forms once spread eastward over Asia because 

 of the single existing species in China thousands of miles from any 

 of its North American relatives but the former continent has been 

 little explored and no records are known. 



Although the Ice Age exterminated the hickories throughout 

 Europe and most of Asia they survived safely in North America 

 and are in no danger except from the axe of the woodman. Their 

 great tolerance of shade and their ability to respond to the stimu- 

 lus of increased light combined with their longevity are important 

 factors in their continued existence. Although rodents consume 

 many of their fruits they have probably done so throughout their 

 whole history, for nuts gnawed by squirrels are not infrequent in 

 Pleistocene deposits. This is not an unmixed evil for various 

 rodents not only distribute the nuts but bury them in afterward 

 forgotten places where they are almost sure to grow. Before the 

 advent of the ''civilized axe" many venerable old giants were scat- 

 tered through our American forests, and there are numerous records 

 of immense trunks showing 350 or more annual rings. There is 

 a record of a pecan at Evansville, Indiana which had a trunk 6 

 feet in diameter showing 400 growth rings showing that it was 

 already a small tree before Cortez conquered Mexico and had 

 reached a considerable size by the time the Pilgrims landed at 

 Plymouth. 



