198 WARREN UPHAM, M.A., F.G.S.A., ON THE NEBULAR AND 



" Without attempting to fix the precise stage, it is not unreason- 

 able to assume that surface waters had begun their accumulation 

 upon the earth's exterior while yet it lay 1,500 to 1,800 miles 

 below the present surface. The present difference between the 

 radii of the oceanic basins and the radii of the continental platforms 

 is scarcely 3 miles, on the average; so that if the continental 

 segments be assumed to be in approximate hydrostatic equilibrium 

 with the oceanic segments to-day, as seems highly probable, the 

 selective weathering process brought about a difference in depres- 

 sion of only 1 mile in 500 or 600 miles, or about one-fifth of 

 1 per cent. . . . 



" Not only is the evolution of the great abysmal basins and of 

 the continental platforms thus assigned to a very simple and 

 inevitable process, but there is therein laid the foundation for 

 subsequent deformation of the abysmal and continental type. 



" . . . A theoretical scantiness of time for a prolonged 

 evolution previous to the Cambrian period has been deduced from 

 a molten earth, but this does not apply to the planetesimal 

 hypothesis. The supposed limitation of the sun's thermal endurance 

 would apply if the arguments could be trusted, but their foundation 

 has been cut away by recent discoveries. It is not the least of the 

 virtues of the planetesimal hypothesis that it opens the way to a 

 study of the problem of the genesis and early evolution of life free 

 from the duress of excessive time limits and of other theoretical 

 hamperings, and leaves the solution to be sought untrammeled, 

 except by the conditions inherent in the problem itself, which are 

 surely grave enough. 



" It is assumed that the conditions on which life is now 

 dependent were prerequisites to its introduction. As already 

 indicated, an atmosphere and hydrosphere sufficient to sustain life 

 may have been acquired when the earth was about the size of 

 Mars, or one-tenth grown. If, to be conservative, a preliminary 

 growth of twice this amount be allowed, there still remains between 

 this and the Cambrian record the growth of four-fifths of the 

 mass of the earth. So far, therefore, as atmosphere and hydro- 

 sphere are concerned, life may have been introduced early in the 

 history of the earth, and may have had a vast interval for develop- 

 ment previous to the earliest legible record. There is another 

 essential condition — a sufficiency, but not an excess, of heat and 

 light. If the formation of the parent nebula involved only the 

 outshooting of a small fraction of the ancestral sun, the solar 

 supply of heat and light may not have been so seriously disturbed 

 as to have fatally affected its availability to furnish what was 

 necessary for life at any stage of the earth's growth. . . 



" . . . There is little ground for apprehension that the 

 infalling planetesimals would be seriously dangerous to the early 

 forms of life, for in the first place the atmosphere must have been 



