10 Illinois Natural History Survey Circular 49 



other. Instead, conditions in the intervening areas must at some time 

 have been different from what they are nov^^, different in a way that 

 would permit the skunk cabbage to disperse across the area between 

 China and eastern North America. The simplest conditions permit- 

 ting such a dispersal are as follows: (1) at some time in the past, 

 temperate climatic conditions favoring the skunk cabbage occurred 

 farther north than they do today, so that the skunk cabbage occurred 

 north to the Bering Straits area in either Asia or North America; (2) 

 the Bering Straits area itself was above water, also temperate, and 

 afforded a land connection over which the skunk cabbage could dis- 

 perse; and (3) since that time, temperate climates of the Bering 

 Straits area have been displaced by colder climates, the temperate 

 climates have moved southward to at least their present positions, and 

 the skunk cabbage has moved with the climatic conditions favorable 

 to it. 



The insect example is a little aquatic caddisfly called Wormaldia 

 mohri (Ross), which is known only from the Great Smoky Mountains 

 of Tennessee and North Carolina. Its only close relatives are Asiatic, 

 and its sister species is now known only in Japan. Because these 

 caddisflies can fly only short distances, the only logical conclusion to 

 be drawn is that the common ancestor of the American and Japanese 

 species had a range extending from eastern Asia into eastern North 

 America. 



From fossil evidence and evidence of relationships and dispersal 

 of living organisms, we have been able to piece together a reliable 

 picture of many historical facets of the plant and animal life in many 

 parts of the world. When this picture is outlined for Illinois, it 

 becomes obvious that we know much about some periods and little 

 about others. 



THE CALENDAR OF LIFE 



In a history, it is difficult to know what to consider as the begin- 

 ning. The areas now occupied by the Dunesland probably came into 

 existence soon after the earth solidified four or five billion years ago. 

 The life of the Dunesland traces back to the origin of life itself, 

 which is estimated to be at least two billion years ago. A fairly con- 

 tinuous and well-preserved fossil record is known for about the last 

 600 million years of the earth's history, although parts of this record 

 are fragmentary. 



At various times during the period chronicled by the fossil 

 record, certain marked changes have been noticed in the physical 

 features and the life of the earth. On the basis of these changes, 



