no such mishap occurs as this, and if the recurrence of the conditions is sufficiently perfect, a series of generations follows with as much certainty as a series of seasons follows upon the cycle of the relations between the earth and sun. Let the first periodically recurring substance--we will say A--be able to recur or reproduce itself, not once only, but many times over, as A1, A2, &c.; let A also have consciousness and a sense of self-interest, which qualities must, _ex hypothesi_, be reproduced in each one of its offspring; let these get placed in circumstances which differ sufficiently to destroy the cycle in theory without doing so practically--that is to say, to reduce the rotation to a spiral, but to a spiral with so little deviation from perfect cycularity as for each revolution to appear practically a cycle, though after many revolutions the deviation becomes perceptible; then some such differentiations of animal and vegetable life as we actually see follow as matters of course. A1 and A2 have a sense of self-interest as A had, but they are not precisely in circumstances similar to A's, nor, it may be, to each other's; they will therefore act somewhat differently, and every living being is modified by a change of action. Having become modified, they follow the spirit of A's action more essentially in begetting a creature like themselves than in begetting one like A; for the essence of A's act was not the reproduction of A, but the reproduction of a creature like the one from which it sprung--that is to say, a creature bearing traces in its body of the main influences that have worked upon its parent. Within the cycle of reproduction there are cycles upon cycles in the life of each individual, whether animal or plant. Observe the action of our lungs and heart, how regular it is, and how a cycle having been once established, it is repeated many millions of times in an individual of average health and longevity. Remember also that it is this periodicity--this inevitable tendency of all atoms in combination to repeat any combination which they have once repeated, unless forcibly prevented from doing so--which alone renders nine-tenths of our mechanical inventions of practical use to us. There is not internal periodicity about a hammer or a saw, but there is in the steam-engine or watermill when once set in motion. The actions of these machines recur in a regular series, at regular intervals, with the unerringness of circulating decimals.