CHAPTER
photographs of these fossils from the base of the series to the top, and copy them on a cinematograph film we could see evolution taking place before our eyes.
Let us remember that this change of one species into another took thousands of years, probably hundreds of thousands; then we shall not expect to find evidence that similar changes have taken place during the brief span of historical time.
One should note also, both in this case and in others that are equally well established, how very regular is the course of the change. A drunken man staggers along, veering from one side of the road to the other, stumbling and stopping at random. His aim is not visible; his course cannot be foreseen. How different is the flight of an arrow towards its mark; rising from the archer's bow, and then sinking in one gentle unbroken curve till it pierces the bull's-eye! This mathematical regularity is due to the momentum imparted by the bow and to the pull of gravity; any deflections due to the wind can be allowed for and calculated. Of like nature, and no less due to natural causes, is the regular change of an evolving series of animals. But whatever may be its cause, the regularity is such that the palaeontologist can predict the existence of forms still unknown, but required, on his theory, to fill a gap or to extend the series backward. Such forms have often been found in accordance with his prediction. This power of correct prediction is generally held to be the strongest proof of any scientific theory.
For these reasons palaeontologists are bound in honesty to accept evolution; but equally in honesty they must confess that they do not yet know all its laws or all its causes. That they disagree upon what they do not know does not prove their testimony false if they agree upon what they do know.
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