What we know of 'dark stars' has been mainly derived from the observation of stellar systems. They are assumed to be the denizens of a stellar Hades, dim wanderers amid the shades, who 'have had their day, and ceased to be' as suns. In the 'cold obstruction' of these viewless orbs the grand cosmical procession is held to terminate. Their presence attests the downward progress of decay, and gives logical completeness to the argument for development. Yet there are circumstances warning us against too full an assurance that their status is really that of skeletons at the feast of light. They are very frequently found to be in close attendance upon brilliant white stars. Thus intimately, if incongruously, coupled, they circulate and compel circulation in brief periods, as members of systems just, it might be said, out of the shell. What are we to think, for instance, of the obscure body spectroscopically discovered to control the revolutions of the chief star in the Orion trapezium? It is evidently comparable in mass with that imperfectly condensed luminary. Is it credible that it has already traversed all the stages of stellar existence, and cooled down to planetary rank? So violent an assumption should at least not be made without due consideration; and we may more prudently hold our judgment in suspense as to whether globes so circumstanced--and they abound--should be regarded as effete, or as abortive suns.[100] Speculations on the exhaustion of stellar vitality have lately become inextricably involved with the complex problem of elemental evolution. A dim inkling has been acquired of the activity in the universe of obscure forces, availing, we can just see, to falsify many forecasts. The theory, among others, of the dissipation of energy needs to be revised or qualified. Nor was it propounded by Lord Kelvin with dogmatic certainty. He carefully noted the possibility that in 'the great storehouses of creation' reserves of energy might be provided by which the losses incurred through radiation could be, wholly or in part, made good.[101] The anticipated possibility is perhaps realized in the phenomena of radio-activity. But if we inquire how, we are met at the threshold by difficulties connected with the origin of helium. Helium appears to result from the disintegration of radium, its generation being accompanied by the setting free of enormous quantities of energy. Its copious presence, then, argues long-continued and lavish expenditure of heat and light. Yet it is as a constituent of highly primitive orbs that it is chiefly conspicuous. Gaseous nebulæ, too, include immeasurable supplies of it, while it is incompatible with whatever we seem to know about them to suppose that radium at any time entered into their composition.