line of change, we have a body like our moon, dead and sterile. And here our speculations end. What next ensues, no one can say. We are seeking the past history of our planet, not looking into its future.
Now, a paper of this sort should always contain a summary of the steps by which its conclusions have been reached. Beginning with the nebular hypothesis, as it is commonly understood, we saw that it was philosophically strong, was supported by much evidence, and opposed by none. Bringing the spectroscope to bear upon it, we found that true nebula? undoubtedly exist, and that there is tolerably good proof of different degrees of complexity among the fixed stars. Notwithstanding these differences, however, we know that the universe is built throughout of essentially the same materials. In order to bring unity out of this diversity in the constitutions of the heavenly bodies, we arranged a series of development, from nebula to planet. This made it apparent that an evolution of matter from lower to higher stages might have accompanied the formation of planets and suns; an idea which was suggested also by physical analogies, and which had decided elements of philosophical strength. And thus we gave to the nebular hypothesis the somewhat novel form which it has received in our speculations. Without our additions, it could derive no real support from the spectroscopic evidence adduced in its behalf. The known nebulæ are simple, our systems of suns complex. By assuming the evolution of matter, these difficulties cease to exist, and we have a coherent hypothesis, in which the evidence offered by the spectroscope is used to good advantage. To be sure, although it is in harmony with many observed facts, it is open to many objections. And yet we can admit its probability, to a certain extent, without giving it the adherence of actual belief. Such theorizing is profitable, partly because it aids us in making out the limits of our present knowledge, suggests to us new paths of investigation, and, by uniting masses of different ideas, helps the mind to handle more easily the facts and conceptions with which it has to deal.
But, when one is fairly started on a line of thoughts, it is hard to come to an end. Problem after problem, theory after theory, law after law, crowd forward for inspection. If we assume one hypothesis to be true, a hundred others rush in upon the mind, and demand consideration. From every one of these a host of interesting conclusions can be drawn, each suggesting another, until the brain grows weary of action. The present case is no exception to the rule. Objections must be answered, consequences foreseen, demonstrations sought. In an article of this scope few points can receive due attention. Let it then suffice, in closing, to say that science has done so much in the past that we can justly expect almost any achievement in the future. And perhaps, in days yet to come, an evolution of matter may be experimentally be brought about, and our speculations of to-day proved to be not altogether foolish.