236 TREE ANCESTORS 



during those ages. As far as is known at the present time no 

 Hndens have been found in deposits as old as the Upper Cretaceous, 

 and the hnden hne is therefore less ancient than that of the majority 

 of our forest trees, unless possibly the unknown expanse of Asia 

 has Cretaceous lindens hidden somewhere in its bosom. 



The oldest known lindens are found in the Eocene or early Ter- 

 tiary. In the rocks of this age 4 or 5 different species have been 

 discovered and although this number is small the localities where 

 they have been discovered point rather unmistakably to the region 

 where the linden stock originated. None occur south of latitude 

 40° and two come from north of latitude 60°, one of the latter being 

 found as far north as Spitzbergen. A linden was recorded by 

 Heer from the upper Eocene of Grinnell Land within 10° of the North 

 pole, but it seems rather to represent a hazel (Corylus). This 

 species, in part apparently representing a linden is known as Tilia 

 Mahngrcni and a leaf of this far northern form is shown on the 

 accompanying plate. It had typical broad leaves about the size 

 of those of the existing European lime. It has been recorded from 

 rocks which are probably late Eocene in age in both Spitzbergen 

 and Iceland, where it is associated with volcanic rocks (basalts) 

 that seem to have characterized the earlier Tertiary throughout the 

 North Temperate and the Arctic regions. It affords perhaps the 

 most striking illustration of the different climate and floral dis- 

 tribution that is disclosed by a review of the ancestors of any of 

 our trees, and it is almost impossible for us to picture forests of 

 broad leaved temperate trees covering the present perpetually ice 

 clad wastes of the far North, and flourishing where the winter's 

 night lasted for six months. 



A second Eocene species occurs in the Kenai region of Alaska 

 and a third on Sachalin Island off the Asiatic coast immediately 

 north of Japan. A fourth has been found in the early Eocene of 

 Montana. This Eocene distribution would seem to indicate that 

 the linden stock originated somewhere in the far north, but whether 

 actually in the Polar region or in northern North America or north- 

 ern Asia it is impossible to say. The Montana species is some- 

 what older than the other known Eocene forms and this may mean 



