PLANETARY EVOLUTION 



entific evidence would hesitate in provisionally 

 accepting it. Devised to account for a certain 

 limited group of phenomena, it not only ac- 

 counts for these, but also for other groups of 

 phenomena, not considered by its propounders. 

 Facts which on a superficial view appeared as 

 obstacles to the theory have on closer exami- 

 nation turned out to be powerful arguments in 

 its favour. It is sustained by all the facts within 

 our ken, and invalidated by none. And it has 

 so far thriven with the progress of discovery 

 during the past hundred and twenty years, that 

 at the present moment it commands wider as- 

 sent than at any previous time since its first pro- 

 mulgation. 



Of this last statement we find striking con- 

 firmation as we pass beyond the limits of the 

 solar system and seek for evidence in the re- 

 motest depths of stellar space. It is well known 

 that Sir William Herschel supposed certain ir- 

 resolvable nebulae to consist of self-luminous 

 vapour hovering cloud-like in space. Laplace 

 associated this hypothesis with his own theory 

 of planetary evolution ; pointing to the present 

 existence of nebulous masses as confirmatory 

 proof of the past existence of such a nebulous 

 mass as his theory required. According to this 

 view, the irresolvable nebulae are simply starry 

 systems in embryo ; and when our planetary 

 system consisted simply of the sun diffused in 

 293 



