Ross: The Dunesland Heritage of Illinois 15 



In Europe many new mountains arose or older ones were elevated to 

 greater heights ; in Asia the interior was elevated and the highest 

 part of the Himalaya Mountains resulted from these tremendous 

 movements; and in North America the entire central area was ele- 

 vated and some of the mountain ranges were formed. These upheavals 

 produced arid conditions and rain shadows that broke the previously 

 continuous temperate deciduous forest into isolated areas. This tem- 

 perate deciduous forest can exist only with a fairly high minimum 

 of rainfall; hence, wherever the rain supply was reduced the hard- 

 wood trees disappeared and other kinds of vegetation took their 

 place. The pattern of rainfall changes that accompanied the crustal 

 unrest of the later Cenozoic Era restricted the deciduous forest to 

 three large areas, one in western Europe, one in China, and one in 

 eastern North America, plus small isolated remnants in central Eur- 

 asia and western North America. To the best of our knowledge, these 

 areas of typical temperate deciduous forest have never been rejoined 

 since they became separated 15 to 20 million years ago. Since the 

 break-up of this world-wide temperate deciduous forest, each isolated 

 segment has evolved in its own way. 



A part of the evidence for the existence of this temperate de- 

 ciduous forest, world-wide in extent, followed by a break-up into three 

 large areas, is that each of many introduced European and Asiatic 

 trees used for ornamentals is like, but not exactly like, one of our 

 native American trees. The European and Siberian elms, for example, 

 have close relatives here, as do also the European beech, sycamore or 

 plane tree, hazel, linden, alder, maple, and others. The domestic fruit 

 trees, such as apple, cherry, and walnut, all Old World in origin, are 

 remarkably similar to many native North American trees. Each pair 

 of such closely related species is evidence that the parental form of 

 both species occurred across the whole band of the former world-wide 

 forest ; after the forest was broken up, the North American segment 

 of each parental species evolved into one species and the European 

 or Asiatic segment evolved into another. In some instances, the 

 separated populations of a plant species did not change at all, as 

 is true of the eastern skunk cabbage, whose populations in eastern 

 North America are still exactly like those that survived in Asia. In 

 other instances, the population in one place changed very little, but 

 populations in other places changed a great deal. An example is the 

 present-day eastern sycamore of North America, which is remarkably 

 like fossils of the mid-Cenozoic transcontinental sycamore; some of 

 the Eurasian populations of this widespread ancestor have since 

 evolved into different species. 



