[Illustration: Lord Rosse’s nebula] But since Keeler’s discovery there has been a decided turning away of speculation another way. The form of the spiral nebulæ seems to be entirely inconsistent with the theory of an originally globular or disk-shaped nebula condensing around a sun and throwing or leaving off rings, to be subsequently shaped into planets. Some astronomers, indeed, now reject Laplace’s hypothesis _in toto,_ preferring to think that even our solar system originated from a spiral nebula. Since the spiral type prevails among the existing nebulæ, we must make any mechanical theory of the development of stars and planetary systems from them accord with the requirements which that form imposes. A glance at the extraordinary variations upon the spiral which Professor Keeler’s photographs reveal is sufficient to convince one of the difficulty of the task of basing a general theory upon them. In truth, it is much easier to criticize Laplace’s hypothesis than to invent a satisfactory substitute for it. If the spiral nebulæ seem to oppose it there are other nebulæ which appear to support it, and it may be that no one fixed theory can account for all the forms of stellar evolution in the universe. Our particular planetary system may have originated very much as the great French mathematician supposed, while others have undergone, or are now undergoing, a different process of development. There is always a too strong tendency to regard an important new discovery and the theories and speculations based upon it as revolutionizing knowledge, and displacing or overthrowing everything that went before. Upon the plea that “Laplace only made a guess” more recent guesses have been driven to extremes and treated by injudicious exponents as “the solid facts at last.” [Illustration: Wonderful spiral in triangulum]