262 TREE ANCESTORS 



shut off the retreat of the numerous Tertiary forms, Diospyros 

 fared but ill on that continent and most of the species became ex- 

 terminated. In America and Asia, a congenial habitat spread 

 far to the southward of the ice-front and there were no dangerous 

 mountain glaciers across their paths, consequently the persimmons 

 were able to maintain themselves and to spread northward again 

 in the wake of the ice sheet. In America, our common persimmon 

 forsook temporarily its more northern haunts, although it is doubt- 

 ful 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 precipitation 

 than to any great degree of secular change in temperature. 



Much more might be written concerning the geologic history 

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

 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 geological ages and 

 that there is the most complete dependence between their ances- 

 tral history and the geologic, geographic, and climatic history 

 of the earth. 



I have attempted to summarize the history of Diospyros in a 

 graphic way on the accompanying small sketch map of the world. 

 The soHd black indicates the distributional areas of the existing 

 species while the ruled lines indicate the larger areas over which 

 Diospyros extended its range during its geological history. This 

 range was probably more extensive even than is indicated since 

 data are not available for plotting the complete record which will 

 always remain more or less incomplete. 



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