

114 GEOLOGY. 



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 surroundings of the growing earth must have somewhat 

 reduced the loss of heat by radiation into space, and so have made 

 some compensation. 



Autogenic heat and light. — There was, however, a terrestrial 

 source of heat and light of critical importance, namely, that arising 

 from the infall of the plane tesimals. If this infall were at a rate suf- 

 ficient to heat the surface of the earth above 100° 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 pro- 

 hibitive temperature, the rate of infall must almost certainly have 

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

 was diminished; so that, long before the supply was exhausted and 

 growth ceased, the rate must inevitably have fallen below the prohibit- 

 ive 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 

 required for its completion would still be long. 



Transition to complete solar dependence. — In the early stages, 

 the danger seems to be all on the side of too great heat. Even if the 

 sun's heat were much less than now, the heat of planetesimal infall 

 would probably make up the deficiency and more. The infall would 

 continue to be a source of home supply so long as the accretion con- 

 tinued, declining as the supply of planetesimals diminished. This 

 diminution of the supply cleared the space between the earth and the 

 sun, and gradually brought the latter into full function. There would, 

 therefore, be a gradual passage from the partial dependence on the 

 home supply of heat and light, to complete dependence on the solar 

 supply. There is little ground for apprehension that the infalling 

 planetesimals would be seriously dangerous to the early forms of life, 



