676 T. C. CHAMBERLIN 



the old scale will doubtless concede that the proportions of the 

 radioactive scale may be used safely as a means of extending the 

 older scale over the Proterozoic and Archean eras, where its own 

 criteria are not available. Using the radioactive scale, the begin- 

 ning of the Paleozoic may be placed, in round figures, at 4X10* 

 years ago, the beginning of the Proterozoic at 12X10^ years, and 

 the oldest portion of the Archean that has been determined in 

 respect to age, at 16 X 10^ years. Using the old scale, the beginning 

 of the Paleozoic may be placed at 10^ years and — using the radio- 

 active scale for proportionate extension — the beginning of the 

 Proterozoic at 3X10^ years, and the earliest determined Archean 

 at 4 X 10* years. To fill out the total period of life-evolution on the 

 radioactive scale, an allowance of 24X10^ years previous to the 

 earliest determined Archean must be made, making the total life- 

 period 4X10' years. To similarly fill out the total period of life- 

 evolution on the old scale, 6X10* years is to be allowed previous 

 to the earliest determined Archean, and loX 10^ years for the whole 

 life-period. 



In thus using the proportions of biologic evolution as an indica- 

 tion of the period over which the growing stage of the earth was 

 spread, it is to be noticed that, to avoid making the assigned period 

 of planetesimal infall unfairly long by including too much of the 

 tailing-out stage, I shall consider all planetesimals that fell during 

 the last 400,000,000 years (radioactive scale) of the Archean era, 

 and all that have fallen since, 16X10^ years in all, as though they 

 had fallen within the computed period. On the other hand, all 

 that fell during the nuclear stage, i.e., before a definite earth-core 

 was formed, are necessarily excluded from the estimated period of 

 life- evolution, since the conditions were incompatible with life. 

 No doubt the infall during the distinctly nebulous portion of the 

 nuclear stage may have been more rapid than during the biologic 

 stage but that does not concern us in considering the period of 

 biologic activity preceding the earliest age-determined Archean. 

 It is the special merit of the planetesimal hypothesis that it takes 

 due account of biological requirements, as generously interpreted 

 as the leaders in biological inquiry demand. The biological 



