The very condensed sketch now given of such of the discoveries of recent Astronomy as relate to the subject we are discussing will, it is hoped, give some idea both of the work already done and of the number of interesting problems yet remaining to be solved. The most eminent astronomers in every part of the world look forward to the solution of these problems not, perhaps, as of any great value in themselves, but as steps towards a more complete knowledge of our universe as a whole. Their aim is to do for the star-system what Darwin did for the organic world, to discover the processes of change that are at work in the heavens, and to learn how the mysterious nebulæ, the various types of stars, and the clusters and systems of stars are related to each other. As Darwin solved the problem of the origin of organic species from other species, and thus enabled us to understand how the whole of the existing forms of life have been developed out of pre-existing forms, so astronomers hope to be able to solve the problem of the evolution of suns from some earlier stellar types, so as to be able, ultimately, to form some intelligible conception of how the whole stellar universe has come to be what it is. Volumes have already been written on this subject, and many ingenious suggestions and hypotheses have been advanced. But the difficulties are very great; the facts to be co-ordinated are excessively numerous, and they are necessarily only a fragment of an unknown whole. Yet certain definite conclusions have been reached; and the agreement of many independent observers and thinkers on the fundamental principles of stellar evolution seems to assure us that we are progressing, if slowly yet with some established basis of truth, towards the solution of this, the most stupendous scientific problem with which the human intellect has ever attempted to grapple. THE UNITY OF THE STELLAR UNIVERSE During the latter half of the nineteenth century the opinion of astronomers has been tending more and more to the conception that the whole of the visible universe of stars and nebulæ constitutes one complete and closely-related system; and during the last thirty years especially the vast body of facts accumulated by stellar research has so firmly established this view that it is now hardly questioned by any competent authority.