may be that a more careful study will show them to be gaseous, with their spectral lines in a state of transition to the full continuous spectrum; but this is little more than bare conjecture at present; for the published descriptions of these nebulæ are too incomplete to admit of very satisfactory discussion.
This consideration of nebular spectra plunges us at once into a sea of difficulties. We say that the sun and planets were formed by condensation and cooling from incandescent vapors, and hail the nebulæ as confirming this opinion. But could a sun be evoked, by cooling, from a body less hot than itself? Moreover, the sun is known to contain at least sixteen elements and probably many more. Were these developed from a nebula containing only nitrogen and hydrogen? Or did the original nebulæ differ in constitution? All those which the spectroscope has analyzed are chemically alike. We know nothing of any whose constitution differs in this respect from theirs; and, therefore, if we point to them as confirmatory of the nebular hypothesis, we are compelled to ask this portentous question: Did our planet, with all its chemical complexity, arise, by a slow process of evolution, from a glowing cloud of but two familiar gases? Upon our answer to this question depends largely the value of our spectroscopic confirmation of the great hypothesis. The safety of the hypothesis itself is not involved; merely that of this one argument in its favor. We can easily conceive of more complex nebulæ, which could give rise to systems like ours, although we know nothing of them. And, if we interpret the spectra of some nebulæ of the second class as due to gases at very high temperature and pressure, the difficulty regarding the heat of our sun will be easily gotten over.
Let us consider the question suggested, as to the possible evolution of complex from simple matter. It is easy to speak out boldly, in an ex-cathedra manner, and say that an affirmative answer to such a question would be absurd; but dogmatism of this sort is, in the highest sense, unphilosophical and foolish. We do not know but that the evolution of one element from another may be possible, under circumstances over which we have as yet no mastery; indeed, such a view would have many points of probability about it. Although unsupported, it is quite strongly suggested by evidence. The demonstrated unity of force leads us, by analogy, to expect a similar unity of matter; and the many strange and hitherto unexplained relations between the different elements tend to encourage our expectations. These elements, which seem to-day so diverse in character, may be, after all, one in essence. This idea is philosophically strong, but waits for experimental evidence to support it. At present, it can neither be discarded as false, nor accepted as true. But what an addition the proof of such a doctrine would bring to the philosophy of evolution!
Now, although questions like these cannot be settled by any evidence which we are likely to obtain for many years to come, specula-