AND ITS INHABITANTS 3 



systems, since from the distances of even the nearer stars the 

 earth and her sister planets would be hopelessly invisible in the 

 most powerful telescope. We cannot, then, follow into the 

 planetary stage the evolution of other systems comparable to 

 our own. Yet in nebulae, in stars, and in the inherited motions 

 and configurations of our planetary system are clues which 

 pieced together lead up toward the origin of the earth. 



The problem of the origin of the earth is within the domain 

 of scientific investigation, but as yet the pictures which may 

 be drawn are varied. The vague outlines shift and change 

 but become clearer with the growth of knowledge. Where 

 the solution of a problem is not yet definitive and certain, the 

 method of multiple working hypotheses should be used. All 

 facts and theories should be matched to these several hypoth- 

 eses to determine which one of them shall be selected and 

 modified, and which shall meet the fate of the unfit. At the 

 present stage of investigation any one view should not be 

 regarded as established beyond question, even though the 

 assembled evidence seems strongly to support it. In a single 

 presentation, however, all hypotheses cannot be equally treated 

 and each investigator, while recognizing the existence of other 

 hypotheses, may properly emphasize that one which seems 

 to him most in accordance with the various categories of facts 

 and more firmly established inferences. 



The hypotheses of earth origin begin more especially in 

 the astronomic field in the search for initial causes; they end 

 in the geologic field where they dovetail into the known rela- 

 tions. The surviving hypothesis must give a sound explana- 

 tion of those broader terrestrial conditions of atmosphere, 

 hydrosphere, and lithosphere, of ocean basins and continental 

 platforms, which had become established by the beginning of 

 the geologic record. But, although much has been learned, it 

 is still unsettled among geologists as to how far those funda- 

 mental conditions in early geologic times were different from 



