220 TREE ANCESTORS 



ties of Pleistocene times but their ranges were broken up in an 

 astonishing way. In North America they survived in our eastern 

 forests, in the Rockies, and in the Pacific coastal region. Few were 

 left in Europe except in the lands bordering the Mediterranean, 

 Many survived in eastern Asia and many more have since been 

 evolved there, so that at the present time about two-thirds of the 

 existing maples find their home in the last named region. More- 

 over glaciation resulted in the seeming pranks of distribution 

 whereby our familiar eastern moosewood or whistlewood, Acer 

 pennsylvanicmn, belongs to a section of the genus whose ten other 

 species are all confined to the Chinese-Japanese area. Similarly 

 our Rocky Mountain sugar maple, Acer grandidentatum belongs 

 to a section whose 8 other species are all Old World and mostly 

 Mediterranean: our mountain maple, Acer spicatum, and broad- 

 leafed maple, ^cer wacro/>//j//MW, belong to a section whose score 

 of additional species are all inhabitants of the Old World: the 

 Negundos or box-elders, of which there are 3 closely related existing 

 forms have 1 in the East, 1 on the Pacific coast, and the third in 

 Central America. 



Even the foregoing brief statement should convince the most 

 incredulous that we cannot understand the present day distribu- 

 tion of any of our trees without some knowledge of their past his- 

 tory, and when this history approaches the completeness that we 

 hopefully look forward to, it will be possible to explain much that 

 still remains obscure. We can predict a priori that when closely 

 related forms are remote from one another today that their ances- 

 tors occupied intermediate regions, and already the above men- 

 tioned break in the distribution of the box-elders is partially 

 bridged by Tertiary forms in the country between. 



The geological history of the ancestral maples is based upon 

 the fossil remains of the leaves and fruits. Fortunately maple 

 leaves have always had a rather characteristic form, not exactly 

 like the leaves of any other trees, and the earliest ones differ from 

 the modem ones in only minor particulars such as having elongated 

 median lobes just as the young leaves of seedlings do at the present 

 time. The winged fruits or maple keys (samaras) are also and 



