Page:Popular Science Monthly Volume 10.djvu/250

238 Dr. Taylor tells us that the Copernican theory "rests on facts presently occurring before our eyes." So does the Ptolemaic theory; and not only that, but, if the test is what occurs before our eyes, then the Ptolemaic theory is a thousand times stronger than the Copernican. If the Copernican theory is so obvious, if it "rests on facts presently occurring before our eyes," why did the astronomers of twenty centuries fail to discern it? Why could not the divines of Copernicus's time see it when it was pointed out to them? And why could not Lord Bacon admit it a hundred years after Copernicus? Dr. Taylor says, "It is verified every day by occurrences that happen according to its laws." So was its opposite, the Ptolemaic theory. Our reverend logician says, furthermore: "We know that astronomy is true, because we are verifying its conclusions every day of our lives, on land and on sea. We set our clocks according to its conclusions, and navigate our ships in accordance with its predictions." And all this they did with the astronomy that preceded Copernicus. Yet the author of this rubbish airs his logical pretensions, and talks about the "effrontery" of Prof. Huxley. Where is his shame?

The Copernican theory is held as demonstrably true, but it is not because everybody can see the demonstration. There was demonstrated truth in the opposite theory, though the proof was not so complete. And in regard to this matter of demonstration, of which so much is said, we have to remember that evidence is a thing of degrees. There may be evidence for a proposition which is so small that we hardly regard its truth as possible—we do not believe it. There is a grade of proof that amounts to probability, but leaves us in uncertainty. There are higher degrees of proof that confer assured belief, and leave little room for doubt. There is, again, evidence so perfect and decisive as to give certainty of conviction, and this we call demonstration. And evidence may have a yet higher shade of intensity, as where we cannot even conceive the opposite of a proposition to be true, and this may be characterized by the frequent expression, "absolute demonstration." The best examples of demonstration are furnished by mathematics in consequence of the fewness and simplicity of mathematical ideas, but demonstration by no means necessarily involves mathematics. There is plenty of demonstrated truth that is not mathematical. The anatomist demonstrates his science by observation, and the chemist by experiment. Fossils found in the rocks demonstrate that life existed upon the globe before the rocks were formed; and the vestiges of art found in Western mounds demonstrate that a race of men superior to the savages formerly lived upon this continent. A theory is said to be demonstrated when it brings all the known facts into agreement, explains them, excludes all other interpretations, and is consistent with itself and all that is understood of the ways of Nature. Most theories of the operations of Nature have about them traces of the imperfection that belongs to all things human, difficulties that are still unresolved, while yet the evidence for them may be so overwhelming as to be held demonstrative.

How is it, now, with the proof of the theory of Evolution, which assumes that the immense diversity of living forms now scattered over the earth has arisen through a long process of gradual unfolding and derivation, within the order of Nature, and by the operation of natural laws? It involves and is built upon a series of demonstrated truths. It is a fact accordant with all observation, and to which there never has been known a solitary exception, that the succession of generations of living things upon earth is by reproduction and genetic connection in the regular order of