The immutable heavens of the ancients strike us to-day as the invention of a strange race of beings. We see them, on the contrary, with Shelley as a 'frail and fading sphere,' a 'brief expanse,' the seat and scene of change. The 'fixed' stars long ago broke away from their moorings, and began to flit at large through space. Of late a less obvious, more intimate kind of mobility has been attributed to them. Grooves of individual development have been assigned to them, along which they appear to shift as the tardy ages go by; and since everything that grows must decay, the orbs of heaven, too, incur the doom of mortality. But modern science has done much more than extend to them the dismal philosophy of the phrase, 'Tout passe, tout casse, tout lasse.' The grandiose enterprise has been not unsuccessfully essayed of tracing in detail the progress of sidereal evolution, and of marshalling the vast stellar battalions in order of seniority. This has been rendered feasible by the disclosures of the spectroscope. Apart from their guidance, the track might have been seen by elusive glimpses, but could never have been laid down with any approach to definiteness. Herschel found for it a _terminus à quo_ in nebulæ of various forms, but attempted to pursue it no further. We do not hesitate to run it on, from station to station, right down to the _terminus ad quem_. Not, it is true, without the perception of outstanding difficulties and insecurities, which yet seem to be outweighed by a certain inevitableness of self-arrangement in the related facts. The argument from continuity is that mainly relied upon. An unbroken succession of instances is strongly persuasive of actual transition, provided only that a principle of development (so to call it) may reasonably be assumed as influential. A series of mineralogical specimens, however finely differenced, does not suggest the progressive enrichment of one original mass of ore. In the stars, on the other hand, a species of vitality may be said to reside. They are not finished-off products, but spontaneously-acting machines. They are centres of energy, which they dispense gratis, supplying the cost out of their own funds. And the process is not only obviously terminable, but must be accompanied by constitutional alterations, which might be traceable by subtle methods of inquiry. They _are_ traceable, unless we are deceived by illusory appearances.