INTRODUCTION 5 



economic importance on account of their relation to coal, oil, and 

 gas. 



How old are fossils? Many people, by various methods, have made 

 estimates, some even guessing at the age of the earth. Geologists 

 may claim that the earth is 10,000 millions of years old, and astrono- 

 mers make no objection. But, whatever the age of the earth may be, 

 it is certain that, in only a comparatively small portion of that time, 

 has there been any life upon it. 



Diagrams are useful, even if they are only estimates (fig. i). In 

 our study of gymnosperms, we have no material below the Devonian, 

 less than one-fifth of the time since the hardening of the original 

 crust of the earth. 



In fact, very little gymnosperm material from the Devonian has 

 been studied. Most of the fossil material has come from the Upper 

 Carboniferous strata, the Coal Measures. One reason why so much 

 material has come from the Upper Carboniferous is that this is the 

 age of coal. It has been called the "Age of Ferns," because the early 

 gymnosperms looked so much like ferns that they were mistaken for 

 them. Comparatively little of the material would have been secured 

 had it not been for the fact that mines have been dug to get the 

 valuable coal, thus uncovering the coal balls and impressions. People 

 would not dig hundreds of feet and spend millions of dollars to get 

 coal balls. They dig the mines to make money, and, as a mere inci- 

 dent, the scientists get the coal balls. Wieland used to say he wished 

 he could make rich people believe that the Permian and the Meso- 

 zoic were really the best places to dig for coal. 



A diagram is very definite, often too definite; but it will help to 

 visualize the situation, and it represents a guess at the real relation- 

 ships of the great gymnosperm groups (fig. 2). 



Most of the historical evidence has come from the university zone 

 of the northern hemisphere. When the rest of the world has been 

 studied, our knowledge will be much more complete; but, as far as 

 great groups are concerned, additional discoveries are not likely to 

 affect the outline of life-histories, but merely to move origins farther 

 and farther back. 



