Now, the point not to be overlooked is this:  to Darwin and Huxley, as to their remote and uncultured ancestors, the World—­the Earth, the Sea, the Sky—­is full of wonders and of mysteries, but the wonders are of a higher order.  The problems of the thunder and of the rainbow as they presented themselves to the men of a thousand generations ago, have been fully solved:  but the questions; what is the veritable nature of electricity, exactly how does it differ from light, are still unanswered.  And what are simple problems like these to the questions:  what is love; why do we feel a sympathy with this person, an antipathy for that; and others of the sort?  Science has made almost infinite advances since pre-historic man first felt the feeble current of intellectual curiosity amid his awe of the storm; it has still to grow almost infinitely before anything like a complete explanation even of external Nature is achieved.

Suppose that, at some future day, all physical and mechanical laws should be found to be direct consequences of a single majestic law, just as all the motions of the planets are (but—­are they?) the direct results of the single law of gravitation.  Gravitation will, probably, soon be explained in terms of some remoter cause, but the reason of that single and ultimate law of the universe which we have imagined would still remain unknown.  Human knowledge will always have limits, and beyond those limits there will always be room for mystery and wonder.  A complete and exhaustive explanation of the world is inconceivable, so long as human powers and capacities remain at all as they now are.

It is important to emphasize such truths, especially in a book addressed to the young.  When a lad hears for the first time that an astronomer, by a simple pointing of his spectroscope, can determine with what velocity a star is approaching the earth, or receding from it, or when he hears that the very shape of the revolving masses of certain stars can be calculated from simple measures of the sort, he is apt to conclude that Science, which has made such astounding advances since the days of Galileo and Newton, must eventually reach a complete explanation of the entire universe.  The conclusion is not unnatural, but it is not correct.  There are limits beyond which Science, in this sense, cannot go.  Its scope is limited.  Beyond its limits there are problems that it cannot solve, mysteries that it cannot explain.

At the present moment, for example, the nature of Force is unknown.  A weight released from the hand drops to the earth.  Exactly what is the nature of the force with which the earth attracts it?  We do not know, but it so happens that it is more than likely that an explanation will be reached in our own day.  Gravity will be explained in terms of some more general forces.  The mystery will be pushed back another step, and yet another and another.  But the progress is not indefinite.  If all the mechanical actions of the entire universe were to be formulated as the results of a single law or cause, the cause of that cause would be still to seek, as has been said.