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



the whole subject of the former speed of rotation of the earth and the 

 relations of the earth to the moon take on a new aspect and invite 

 investigation along the lines of new working hypotheses. Can it be 

 shown that it is absolutely necessary that the aggregating meteoroids 

 gave to the earth an exceedingly high rotation at the outset? Is not 

 this assumption of high rotation merely an offspring of the nebular 

 hypothesis? If the moon were aggregated slowly and came into tidal 

 functions at a late stage, and at a distance from the earth's center quite 

 unknown, may not all its relations to the earth have developed on 

 much more conservative lines than those worked out by Darwin and 

 at the same time preserve those apparently significant relations to the 

 movements of the two bodies to which Darwin has so strongl}^ appealed 

 in support of his hypothesis of the historj^ of the two bodies? In other 

 words, without challenging the validity of Darwin's most beautiful 

 investigation in the essentials of its method, ma}^ not a change in the 

 premises deducible from an equally legitimate hypothesis of the 

 original condition of the two bodies lead to results in equally satis- 

 factory accord with the existing relations of the two l)odies ? 



At any rate, as remarked at the outset, the time limits assigned on 

 tidal grounds are not very restrictive, even on the assumptions made, 

 and when they shall be worked out on revised data in accord with the 

 newer hypotheses they may, perhaps, even be found to favor the 

 longevity of the earth and become one of the arguments in support 

 of it. 



II. 



A third line of argument relative to the habitable era of the earth 

 is drawn from the theoretical age of the sun. After stating tlic prob- 

 ability that, if sunlight was ready, the earth was ready both for vege- 

 table and animal life within a century, or at least a few centuries, after 

 the consolidation of the earth's surface, Lord Kelvin inquires Avhether 

 the sun was ready, and replies:^ 



''The well-founded dynamical theory of the sun's heat, carefully 

 worked out and discussed by Holmholtz, Newcomb, and myself, says 

 no if the consolidation of the earth took place as long [ago] as tifty 

 million years; the solid earth must in that case have waited twenty or 

 fifty [thirty?] million years for the sun to be anvthing nearly as warm 

 as he is at present. If the consolidation of the earth was finished 

 twenty or twenty-five million years ago, the sun was probably ready, 

 though probably not then quite so warm as at pi'esent, yet warm 

 enough to support some kind of vegetable and animal li'fe on the 

 earth." 



Here is an unqualified assumption of the completeness of the Helm- 

 holtzian theory of the sun's heat and of the correctness of deductions 

 drawn from it in relation to the past life of the sun. There is the 



' Science, May 19, 1899, p. 711. 



