On one point we have, nevertheless, acquired certainty. It is now known that comets with their dependent trains of meteors are aboriginal in the solar system. They are no unlicensed intruders, but collateral relations of the planetary family. Possibly they represent waste scraps of world-stuff which escaped the action of the formative machinery; and if so, they exemplify its primitive texture. Not that their composition need be, on this supposition, identical with that of the planets. A sifting of elements would have been likely to accompany the processes of cooling and contraction. Comets were, perhaps, made (to speak illustratively) of the white of the nebulous egg, planets of its yolk. But in any case we may safely regard the glimmering fabrics of acetylene and cyanogen that occasionally illuminate our skies as shearings from a wide-spreading, fleecy haze, flung aside before 'the starry tides' had as yet begun to 'set towards the centre.' In one respect the quality of these relics is a surprise. They show no chemical affinity with nebulæ. Their spectra are radically different from nebular spectra, gaseous or continuous. They accordingly lend no countenance, although not fatally adverse to the view that the sun was once, in the distinctive sense, a nebulous star. The grand topic of sidereal succession is no longer abandoned to fruitless surmises. Broad lines have been laid down, along which, so far as we can at present see, progress must inevitably have been conducted. And one fact of overwhelming significance in this connection is entirely of recent discovery. The multitudinous existence of obscure bodies in space had, indeed, been foreseen as a logical necessity long before Bessel founded the 'Astronomy of the Invisible'; but its strong substantiation is almost wholly due to the use of modern spectrographic methods. Decrepit or dusky suns are assuredly no imaginary product, but a potent reality, though it would be too much to assert that all have sunk to extinction by the same road. Nor is it absolutely certain that their present state is uniformly the outcome of prolonged decay. Circumstances connected with many of them suggest rather a congenital incapacity for shining. We stand, too, on firmer ground than our predecessors in respect to the history of stellar systems. That its course is mainly prescribed by the influence of tidal friction has been ably demonstrated by Dr. See. Telescopic double stars can be led back by the aid of this clue to an initial stage, when they revolved close together, very much like the earth and moon in Professor Darwin's theory; and it was owing to their voluminousness and the unequal attractions it engendered that their orbits became enlarged and elongated to the degree generally observed.