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THE AMERICAN NATURALIST [Vol. XL VI 



bearing formations present are characterized as follows : 

 The Kootenai by 120 species, the Colorado by perhaps 50 

 species, the Dakota by 460 species, the Montana by 150 

 species, the Laramie by 140 species, the Arapahoe by 30 

 species, the Denver by more than 150 species, the Fort 

 Union by from 500 to 700 species, etc., etc. This shows 

 that, as Professor J. W. Judd once said : "¥e still regard 

 fossils as the 'Medals of Creation,' and certain types of 

 life we take to be as truly characteristic of definite periods 

 as the coins which bear the image and superscription of 

 a Eoman emperor or of a Saxon king. ' ' 



Just a word may be said on the economic application 

 of stratigraphic paleontology. It is perhaps safe to say 

 that never in the history of American geology has there 

 been so close an interrelation and dependence of geology 

 on paleontology as at present, and of this confidence 

 paleobotany may justly claim its full share. Thus, of the 

 even dozen of paleontologists in the employ of the U. S. 

 Geological Survey and covering all branches of the sub- 

 ject, four are paleobotanists. 



Among the many subsidiary problems connected with 

 the application of paleobotany to geology, the use of 

 fossil plants as indices of past climate occupies a most 

 important place. As the majority of plants are attached 

 to the substratum and hence are unable to migrate like 

 most animals when the temperature of their habitat be- 

 comes unfavorable, they must either give way or adapt 

 themselves gradually to the changed conditions of their 

 environment. Tl^-efore, fossil plants have always been 

 accorded first place as indices of past climates. "They 

 are," as Dr. Asa Gray has said, "the thermometers of 

 the ages, by which climatic extremes and climate in gen- 

 eral through long periods are best measured." 



To those who have not given especial consideration to 

 the subject, the idea appears to obtain that climatic varia- 

 tions, such as now exist, are normal or essential and that 

 they were present without marked differences during all 

 geological ages. It is now established, however, that this 



