CHAPTER IV 

 THE AGE OF REPTILES 



Geologists divide the history of the earth, since life first appeared 

 upon it, into four general eras, the Proterozoic, Paleozoic, Mesozoic, 

 and Cenozoic, that is, into eras of first life, ancient life, middle 

 life, and recent life. These divisions were made long ago by geolo- 

 gists when it was believed that extraordinary changes, great cata- 

 clysmic revolutions, marked their limits. 



With a fuller knowledge of the life of the past we know that 

 evolution has been continuous and uninterrupted; possibly acceler- 

 ated or retarded at times, but without break. Were the earth's 

 history to be written anew, with our present knowledge, and with an 

 unbiased mind, it is very doubtful whether many of the time 

 divisions would have the same limits that they have now — whether 

 the Paleozoic would terminate with the Carboniferous, or the 

 Permian, or the Trias, or whether indeed we should think it neces- 

 sary to make any primary divisions whatsoever. In other words, 

 our greater knowledge of living and extinct organisms, and of the 

 rocks which contain fossils, has made the problems of classification 

 much more complex than they seemed to be formerly. It is much 

 easier to classify organisms or rocks, or anything else, when we 

 know only a few isolated kinds — much easier to draw divisional 

 lines. Geological history is like a volume in which pages, leaves, 

 and even whole chapters either are missing or are printed in lan- 

 guages which we understand only imperfectly. Where the lost or 

 unknown parts belong, the largest divisions may be made, and 

 possibly such may have been epochs of unusual activity, of dias- 

 trophic changes which greatly accelerated organic evolution. No 

 one can say just where the dividing line should be drawn between 

 the rocks of Paleozoic and Mesozoic age, or between the Mesozoic 

 and Cenozoic, for there is none; the most that we can hope for is to 

 make the divisions everywhere in the world conform to those first 

 made for local reasons. 



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