le 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.

When we bear in mind, then, the omnipresence of this tendency in the
world around us, the absolute freedom from exception which attends its
action, the mann