THE ORIGIN OF THE EARTH, 51 



While awaiting, for how long we cannot tell, such positive deter- 

 mination, it may contribute to clearness of view, even if it shall fail 

 to be an anticipation of the real fact, to definitely picture the possible 

 origin of the supposed nebula that grew into the present solar system. 



The hypothetical origin of the solar nebula. 



An implied ancestral system. — The inference that a spiral nebula 

 is formed by a combined outward and rotatory movement implies 

 a preexisting body that embraced the whole mass. In harmony with 

 this, an ancestral solar system is postulated — a system perhaps in no 

 essential respect different from the present one. Our hypothesis does 

 not, therefore, concern itself with the primary origin of the sun, or of 

 the stars, or with the ulterior questions of cosmic evolution. It con- 

 fines itself to a supposed episode of the sun's history, in which the 

 present family of planets had its origin, and in the initiation of which 

 a possible previous family may have been dispersed ; but no affirmation 

 is made relative to the last point. With some partiality perhaps, this 

 episode may be regarded as geologic, since it specially concerns the 

 birth of the planet of which alone we have intimate knowledge. 



The question of adequacy of energy. — To this conception of an 

 ancestral sun, with an undefined antecedent history as a star, question 

 will arise at once as to a sufficiency of energy for the sun's mainte- 

 nance through such a prolonged history. It has been strongly urged 

 during the past half-century, by very eminent physicists, that the 

 resources of energy assignable for the maintenance of the sun's heat 

 and light could, at best, be barely sufficient for the geological and 

 biological demands of the earth's known history, even when these 

 are most conservatively estimated. How much less then can they 

 be sufficient for an antecedent history of unknown duration! This 

 objection is based on the assumption that the sun's heat and light 

 are derived almost wholly from self-compression, as urged by Helm- 

 holtz. That self-compression is a potent source of heat is not ques- 

 tioned, but the Helmholtzian theory takes no account of sub-molec- 

 ular and sub-atomic sources of energy. The transcendent potency 

 of these sources of energy has been for some time suspected, 1 and is 



1 The following was written in 1899 before experimental demonstration had been 

 reached: "Without questioning its correctness, is it safe to assume that the Helm- 

 holtzian hypothesis of the heat of the sun is a complete theory? Is present knowledge 



