28 THE ORIGIN AND EVOLUTION OF LIFE 



lution, being the first to point out that the high degree of evo- 

 lution and speciaHzation seen in the invertebrate fossils at the 

 very base of the Palaeozoic was in itself a proof that pre-Palaeo- 

 zoic evolution occupied a period as long as or even longer than 

 the post-Palseozoic. In 1869 Huxley renewed this demand for 

 an enormous stretch of pre-Palaeozoic or pre-Cambrian time; 

 and as recently as 1896 Poulton^ found that 400,000,000 years, 

 the greater limit of Kelvin's original estimate, was none too 

 much. 



Later physical computations greatly exceeded this biological 

 demand, for in 1908 Rutherford- estimated the time required 

 for the accumulation of the radium content of a uranium min- 

 eral found in the Glastonbury granitic gneiss of the Early 

 Cambrian as no less than 500,000,000 years. This physical 

 estimate of the age of the Early Cambrian is eighteen times as 

 great as that attained by Walcott'' in 1893 from his purely 

 geologic computation of the time rates of deposition and max- 

 imum thickness of strata from the base of the Cambrian up- 

 ward; but recent advances in our knowledge of the radioactive 

 elements preclude the possibility of any trustworthy deter- 

 mination of the age of the elements through the methods sug- 

 gested by Joly and Rutherford. 



We thus return to the estimates based upon the time 

 required for the deposition of sediments as by far the most 

 reliable, especially for our quest of the beginning of the life 

 period, because erosion and sedimentation imply conditions of 

 the earth, of the water, and of the atmosphere more or less 

 comparable to those under which life is known to exist. These 

 geologic estimates, which begin with that of John Phillips in 

 i860, may be tabulated as follows: 



^ Poulton, Edward B., 1896, p. 808. - Rutherford, Sir Ernest, 1906, p. i8g. 



^ Walcott, Charles D., 1893, p. 675. 



