EVIDENCES OF PEIMITIVE LIFE. 



By Charles D. Walcott. 



[With IS plates.] 

 INTRODUCTION. 



Few of us have a clear realization of the age of the earth. Under 

 many deceptive aspects she carries ^Yith her the secrets of a long 

 and bnsy life, one of such fascinating activity that it is not sur- 

 prising that students are ever seeking to unravel the mj^steries of 

 the past. With all the evidences of youth there is to be felt, espe- 

 cially among the mountains, a sense of age and infinite power, and 

 we are inspired with awe as we trace the base of worn-down rocks, 

 miles in thickness, that formed the mountain ranges far back in 

 geologic time. 



The age of the earth in years I shall not attempt to discuss. A 

 recent resume^ shows the relative age of the sedimentary strata 

 for each period of its histo^3^ These figures point to a minimum 

 time limit of scarcely less than 90,000,000 of years since water and 

 wind began to transport continental earth and rocks over the land 

 and into seas and lakes. How long before that the earth history 

 began it is difficult even to conjecture. With the discovery of the 

 stored-up energy of radium and the development of the planetesimal 

 hypothesis by Dr. T. C. Chamberlin, the supposed, fixed standards 

 of the past generation have been swept away nnd new conceptions 

 are being slowly formulated and subjected to all the tests that mod- 

 ern earth science can conceive. 



A concrete conception of the age of life on the earth is suggested 

 by recalling that the Cambrian sj^stem, with its enrlj and semiprimi- 

 tive forms of invertebrate marine fossils, stands, roughly speaking, 

 midway in the earth's history; approximately as long a period of 

 time was required to develop life to the Cambrian stage of evolution 

 as has since elapsed up to the present time. 



My own investigations have been mainly in the Cambrian and pre- 

 Cambrian strata and have involved new and somewhat startling 



1 M. Joly : The Age of the Earth, Ann. Rept. Smithsonian Inst.. 1011, Washin-rton. 1912, 

 pp. 271-293. 



235 



