The Theories do not account for the Amount of Heat required 106 Evolution of Matter 107 Objection considered 109 Can we on Scientific grounds trace back the Evolution of the Universe to an Absolute First condition? 110 ------------------------------------------------------------------------ STELLAR EVOLUTION. [Illustration] PART I. _THE IMPACT THEORY OF STELLAR EVOLUTION._ Upwards of twenty years ago[1] the theory—or, I should rather say, the hypothesis—was advanced[2] that our sun was formed from a hot gaseous nebula produced by the colliding of two dark stellar masses; and that, as the stars are suns like our own, they in all likelihood had a similar origin. The probability of this theory has been very much strengthened by the facts, both astronomical and physical, which have accumulated since the theory was enunciated. Before proceeding to the consideration of these facts, and the conclusions to which they lead, it will be necessary to give a statement of the fundamental principles of the theory. [Footnote 1: _Philosophical Magazine_, May 1868; _Climate and Time_, chap. xxi.; _Quarterly Journal of Science_, July 1877; _Phil. Mag._, July 1878; _Climate and Cosmology_, chaps. xvii. xviii. and xix.] [Footnote 2: I prefer to use the term “theory,” with the above understood qualification, viz. a theory in its hypothetical stage.] In the theory here discussed the truth of the nebular hypothesis, which begins by assuming the existence of a solar nebulous mass, is taken for granted. The present theory deals not so much with the nebulous mass itself as with the _formation_ of the nebula, and with those causes which led to its formation. For convenience of reference, and to prevent confusion, I have called it the “Impact Theory,” by which name it may be distinguished, on the one hand, from the nebular theory, and, on the other hand, from the meteoric theory, and all other theories which regard gravitation as the primary source of the solar energy.