20 



EDWARD W. BERRY 



mons were able to maintain themselves and to spread northward 

 again in the wake of the ice sheet. In America, our common per- 

 simmon forsook temporarily its more northern haunts, although 

 it is doubtful if its northern limit at any time was farther south 

 than the Potomac River, since it is extremely probable that the 

 extensive Pleistocene glaciation was due more to unbalanced pre- 

 cipitation than to any great degree of secular change in tempera- 

 ture. 



Much more might be written concerning the geologic history 

 of Diospyros and its migrations in the past, as well as something 



V-.. 



,J^3- 



9s 



flflrn^'-' 





Fig. 7 Sketch map of the world showing the distributional areas of the existing 

 species of Diospyros (solid black) and the more extended range of the fossil species 

 (ruled lines). 



of the existing species, — their utility, beauty, and habits of life, 

 but enough has been recorded here to show how immensely remote 

 its forbears were and what an extensive territory its ancestors 

 once occupied. ' In closing, let me repeat the cardinal fact first 

 emphasized in Alfred Russel Wallace's work on distribution, 

 that the present day geographical distribution of plants is almost 

 entirely the end product of their distribution in antecedent geolog- 

 ical ages and that there is the most complete dependence between 



