HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 113 



it to some distant and unknown body. Nor is the problem vexed by 

 duress of severe time-limits. 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 accretion 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 (Vol. I, pp. 556-557). It is not the least 

 of the virtues of the accretion 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 hamper- 

 ings, and leaves the solution to be sought untrammeled, except by 

 the conditions inherent in the problem itself, which are surely grave 

 enough. 



The agency and the mode of genesis of terrestrial life are primarily 

 biological questions, and as biology has, as yet, no solution to offer, 

 these greatest of problems will not be considered here. The specially 

 geological questions relate to the stage at which the requisite condi- 

 tions for life were provided, to the special features of those conditions, 

 and to the period available for evolution previous to the Cambrian 

 records. 



When were the conditions requisite for life first provided ? — It is 

 assumed that the conditions on which life is now dependent were pre- 

 requisites to its introduction. As already indicated, an atmosphere 

 and a 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 hydrosphere are concerned, life may have been intro- 

 duced early in the history of the earth, and may have had a vast interval 

 for development 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 ability to furnish what was necessary for life 

 at any stage of the earth's growth. The planetesimals between the 

 earth and the sun, during the early stages before they were much swept 



