114 TREE ANCESTORS 



That these methods are effective is shown by the fact that the 

 alder has penetrated farther into South America than ahnost any- 

 other of our northern trees, and we can merely guess at the vast 

 lapse of time represented by such a journey from Greenland to 

 BoUvia. 



The alders are unobtrusive modest trees, very tolerant of shade 

 from their youth, consequently gregarious and successful. Some 

 of them reach farther north at the present time than any other 

 members of the family except the small Arctic birches, and they are 

 the only representatives of this large family that cross the equatorial 

 zone and maintain themselves in the Southern Hemisphere. 



They are found in existing floras from Alaska to Labrador and 

 southward to northern Florida and the uplands of Mexico and 

 Central America and down the Andes through Colombia, Ecuador 

 and Peru to BoHvia. That this extension into South America 

 was not due entirely to the stimulus of northern glaciation is shown 

 by the presence of a Pliocene (i.e., a preglacial) species in Bolivia. 

 In the Old World alders are found from Kamchatka to Spain, and 

 from Norway to Algiers and Assam, being present in all the inter- 

 vening great mountain systems. 



The earhest supposed representatives of the alder consist of 

 "Upper Cretaceous forms from western Greenland, Silesia, western 

 Canada, and the Dakota sandstone of Minnesota and Nebraska — 



Fig. 24. Some Fossil Birches (About f Natural Size) 



1. Belnlites Westii latijolius Lesq., from the Dakota sandstone of Kansas. 



2. 2a. Leaves of the Arctic birch, Bdula nana, from the Interglacial of 

 Denmark. 



3. Betula grandifolia Ettings., from the Upper Eocene of Alaska. 



4. Betula coryloides Ward, from the early Eocene of Montana. 



5. Betula oxydonta Sap., from the Oligocene of France. 



6. Betula helcromorpha Knowlton, from the Eocene of Oregon. 



7. 7a, 7b. Leaf and fruits of Betula prisca Ettings., from Miocene of Austria. 



8. Betula Brongniarti Ettings., from the Miocene of Austria. 



9. 9a. Leaf and fruit of Betula cuspidens Sap., from the Upper Oligocene of 

 France. 



10. Fruit of Betula dryadum Brongn., from the Miocene of Europe. 



