FUNDAMENTAL PROBLEMS OF GEOLOGY. 215 



distances at which these nebulae seem to be placed — no parallax hav- 

 ing been as yet detected — changes of position must necessarily be 

 slow in revealing themselves to observation. It is to be hoped, 

 however, that the present rapid progress in the perfection of instru- 

 ments and of skill will soon bring within the reach of successful 

 study some of the smaller spiral nebulae that represent the solar 

 system more nearly in mass and proportions. 



With this much of knowledge and of limitation of knowledge rela- 

 tive to existing nebulae, the construction of a working hypothesis 

 required not a little resort to supplementary deductive and hypo- 

 thetical considerations. The inference that a spiral nebula is formed 

 by a combined outward and rotatory movement implies a preexist- 

 ing body that embraced the whole mass. In harmony with this, an 

 ancestral solar system has been postulated — a system perhaps in 

 no very essential respect different from the present one. My 

 hypothesis does not, therefore, concern itself with the primary 

 origin of the sun or of the stars, or w^ith the ulterior questions of 

 cosmic evolution. It confines 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 this. 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. 



To this conception of an ancestral sun with an undefined ante- 

 cedent history as a star, question will arise at once as to a sufii- 

 ciency of energy for the sun's maintenance 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 

 sufl&cient for the geological and biological demands of the earth's 

 known history, even when these are most conservatively estimated; 

 how much less then can the)' 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 zuholly from self- 

 compression, as urged by Helmholtz. This self-compression has 

 usually been computed on the basis of certain limiting assumptions, 

 the validity of which is open to question. 



That self-compression is a potent source of heat is not doubted, 

 but the Helmholtzian theory takes no account of sub-molecular and 

 sub-atomatic sources of energy. The transcendent potency of these 



