What is cosmic evolution? In all the hypotheses which astronomical physics has imagined we see the transformation of a system--a part of the universe arbitrarily detached from all the rest--through a series of stages, each phase of the series being marked by a progressive decrease of diversity, that is, by some degradation of energy. Two main series of hypotheses accounting for the present condition of the universe seem to have been the result of physical investigation: (1) the origin of discrete solar and planetary bodies by a process of condensation of a gaseous nebular substance; and (2) the origin of the same systems by aggregations of meteoric dust. Plausible as is the nebular hypothesis on first consideration, it fails when it is subjected to minute analysis. What is a gaseous nebula? It is a mass of heated vapour contracting by the mutual gravity of its parts as its molecules lose their heat by radiation--so the hypothesis states. But it has been pointed out that we cannot be certain that the gaseous nebulæ known to astronomy are hot, or even that they gravitate. The great nebula in Orion, it is stated, is at an enormous distance from us, and making a minimal estimate of this distance the volume of the nebula must still be incredibly great. There are good reasons for believing that the mass of the visible universe cannot be greater than that of a thousand million of suns such as our own. Assuming that all this matter is contained in the great nebula in Orion (and obviously only a small portion of it can be so contained), we find on calculation that the “gas” so formed would be much less dense than even the trace of gas contained in a high vacuum artificially produced.[33] How, then, can we speak of such a body as this nebula as an extended mass of hot gas, cooling and gravitating as it loses heat? [33] Its density would be 1/(58 × 10^8)th that of our atmosphere.