It will be observed that the theory here begins in the middle of a process. It begins with the assumption of a mass in the act of condensing under the influence of gravity. It offers no explanation of the origin of the mass, or how it came to be in this attenuated state, or in what condition it existed before the materials began to draw together. These are, however, inquiries which naturally force themselves on our attention. If the nebular theory be a true theory of the origin of the solar system, then this nebulous mass must have had an antecedent history, and we cannot help feeling the instinctive desire of tracing the chain of causation farther back. The mind presses towards an absolute beginning. It is the goal to which it aspires, and no amount of failure will ever deter it from renewing its efforts. Of recent years a considerable amount of attention has been devoted to inquiries in this direction; nearly all of which, it is true, has necessarily been of a speculative and hypothetical character. But hypothesis, as Mr. Locyker remarks, is the life-blood of investigation. The nebular hypothesis is so highly probable as to have gained almost universal acceptance. In fact, it contains very little of a hypothetical nature. It is, as Mr. Mill says, “an example of legitimate reasoning from a present effect to its past cause, according to the known laws of that cause.” Like the hypothesis of a luminiferous ether, if it is not a true theory, one would almost think that it deserves to be so. There seems no reason why inquiries should stop at the point where Laplace began. The same line of reasoning may yet carry us back into the pre-nebular region, and perhaps with as great a degree of certainty as it has done in the nebular; though, no doubt, the farther back we proceed, the more difficult probably will the inquiry become. But, be all this as it may, there can be little doubt that the path of investigation is a legitimate one, and also one which is worthy of being traced out. I shall now briefly refer to some of the leading views which have been expressed in regard to the pre-nebular history of the universe, and shall afterwards consider the additional light which the theory discussed in this volume seems to cast on the subject. The commonly received opinion is that the nebulæ were formed from ordinary matter existing in a high state of division, and widely diffused through space. The “cosmical dust,” as it is called, was the universal “world-stuff,” out of which all things were supposed to be formed. It is held that in receding backwards in pre-nebular times, the smaller, more simple, and elementary the materials were. Out of this primitive cosmical dust, or world-stuff, by aggregation, the materials became successively larger and more complex. The theory of the origin of nebulæ, on this principle, has been clearly stated by Professor Winchell, and I here give a brief outline of his views on the subject.