This is so important that it may be well to make the idea clearer by an illustration. The earth revolves round the sun in its annual orbit under the influence of two forces: the centripetal, or force of gravity tending to draw it towards the sun; and the centrifugal, tending to make it dart away into infinite space. During half the orbit the centripetal seems to be gaining ground on the centrifugal, and the earth is approaching nearer to the sun. If this continued it would revolve ever nearer and soon fall into it; but the centrifugal force is gradually recruiting its strength from the increased velocity of the earth, until it first equals the centripetal, and finally outstrips it, and for the remaining half of the orbit it is constantly gaining ground. If this went on, the earth would fly off into the chilly regions of outer space; but the centripetal force in its turn regains the ascendency; and thus by the balance of the two forces our planet describes the beautiful ellipse, its harmonious orbit as a habitable globe; while comets in which one or the other force unduly preponderates for long periods are alternately drawn into fiery proximity to the sun, and sent careering through regions void of heat. Compare this passage from Herbert Spencer: ‘As from antagonist physical forces, as from antagonist emotions in each man, so from the antagonist social tendencies man’s emotions create, there always results not a medium state, but a rhythm between opposite states. The one force or tendency is not continuously counterbalanced by the other force or tendency; but now the one greatly preponderates, and presently by reaction there comes a preponderance of the other.’ And again: ‘There is nowhere a balanced judgment and a balanced action, but always a cancelling of one another by opposite errors. Men pair off in insane parties, as Emerson puts it.’ The reader will now begin to understand the sense in which polarity applies to these complex conditions of an advanced evolution. To return, however, from this digression to the point at which it began, viz. the origin of life, we have to show how the law of polarity prevails in the organic as well as in the inorganic world. In the first place the material to which all life is attached, from the speck of protoplasm to the brain of man, is strictly a chemical product of atoms and molecules bound together by the same polar laws as those of inorganic matter.