250 CARNEGIE INSTITUTION OF WASHINGTON. 



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 preliminar>^ growth of twice this 

 amount be allowed, there still remains between this and the Cam- 

 brian 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 introduced 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 seri- 

 ously disturbed as to have fatally affected its availability to furnish 

 what was necessar}^ 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 up by the inner planets, may have 

 screened off some appreciable part of the sun's heat and light, but 

 the ratio of nebular matter to space was probably too small to render 

 this loss critical. So long as the nebula itself remained luminous 

 the nebular light compensated in greater or less degree for the solar 

 light cut off, but perhaps not for the heat. The nebulous surround- 

 ings of the growing earth must have somewhat reduced the loss of 

 heat by radiation into space, and so have made some compensation. 

 There was, however, a terrestrial source of heat and light of crit- 

 ical importance, namely, that arising from the infall of the planet- 

 esimals. If this infall were at a rate sufficient to heat the surface 

 of the earth above ioo°C., life of the present types would have been 

 prohibited. The present stage of the inquiry does not permit any 

 very confident opinion as to whether this excess was reached or not. 

 Leaving this question open, it is to be noted that if, at the stage 

 when first an atmosphere and hydrosphere could be held, the infall 

 of planetesimals was so rapid as to heat the surface to a prohibitive 

 temperature, the rate of infall must almost certainly have declined 

 as the number of planetesimals in the earth's feeding zone was dimin- 

 ished ; so that, long before the supply was exhausted and growth 

 ceased, the rate must inevitably have fallen below the prohibitive 

 limit. If, therefore, the earth were too hot for life when one-fifth 

 grown, its temperature might have become suitably mild when one- 

 fourth, one-third, one-half, or three-fourths grown. Growth after 

 this permissive stage was reached would be slow, and the period re- 

 quired for its completion would still be long. 



