326 THE POPULAR SCIENCE MONTHLY. 



line of change, we have a body like our moon, dead and steiile. 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. Notwith- 

 standing 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 ar- 

 ranged 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 nebu- 

 lar 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 

 nebulse are simple, our systems of suns complex. By assuming the 

 evolution of matter, these difficulties cease to exist, and we have a co- 

 herent 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 adhe- 

 rence 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 considera- 

 tion. From every one of these a host of interesting conclusions can be 

 drawn, each suggesting another, until the brain grows weary of ac- 

 tion. 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 per- 

 haps, in days yet to come, an evolution of matter may be experimen- 

 tally be brought about, and our speculations of to-day proved to be not 

 altogether foolish. 



