244 THE AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



especially oceanic life; indeed, the strokes of the meteorites mig-ht not 

 be more inimical to the perpetuity of any given form of life then than 

 are the attacks of its nmnerous enemies to-da3^ It was only another 

 form of jeopardy. The latitude as to variation of rate of infall would 

 be rather large. The infall must not have been so rapid as to have 

 given a universal surface heat above 100° C. The life of hot springs 

 crowds close upon this upper limit, as Lord Kelvin has indicated. The 

 infall must not have been so slow as to have permitted the surface heat 

 to fall universally below 0° C, making allowance for other sources. 

 These other sources might have permitted the meteoric supply to fall 

 considerabl}' below the quantitj^ represented by a surface temperature 

 of 0'-^ C. Between this indeterminable low point and a supply equiva- 

 lent to 100° C, similarlv qualified, there is a quite wide range. Those 

 who have insisted upon the precipitate infalling of meteorites at such 

 a rate as to reduce the earth to a nebulous condition will probabl}" not 

 feel entitled to doubt the adequacy of this source of light and heat. 

 They can only question the possibility of the meteorites falling in 

 slowly enough to permit the coincident presence of life on the earth. 



This hypothesis starts life at a period when the earth was one-fifth 

 grown and prolongs it throughout the rather slow gathering in of the 

 last four-fifths of the earth's mass, and hence gives to the earth a long 

 era of autogenic life conditions. 



Now, if a hypothesis relative to the early constitution and the 

 growth of the rest of the solar system concordant w^ith this be enter- 

 tained — that is, a constitution of a predominantly meteoroidal rather 

 than a gaseous condition, and of a slow rather than a precipitate 

 aggregation — it will, perhaps, appear that the output of heat by the 

 sun in the stages concurrent with this autogenic life period of the 

 earth jnnj have been small. The autogenic thermal era of the earth 

 maj^ thus have corresponded to a period of slight thermal loss by 

 the sun. 



As time went on the ingathering of the terrestrial meteorites gradu- 

 ally became more and more distant from one another (since the scat- 

 tered material was progressively exhausted by previous infalls), while 

 the central or solar aggregation was yet onl}^ in its earl}^ stages and 

 was gradually increasing in heat. If this increase was in a ratio some- 

 what proportionate to the decline of the autogenic heat of th(^ earth, 

 an equalizing conq^ensation might result, and the earth gradually 

 pass from the relatively independent autogenic thermal stage to the 

 dependent solar stage which has continued to the present. Thus, by 

 the prolonged coincidence of increase on the one side with decrease on 

 the other, the life history of the earth may have been transferred 

 from meteoroidal to solar dependence without such a radical disrup- 

 tion of continuity as to have been generally destructive. 



This speculation may seem at first thought to be far-fetched, and to 



