of the spiral, and to suppose also that these structures are
distinguishable from each other at every section. If we think of the
intersection of these with the rising surface, as the atoms, or
physical units, of a plane universe, we shall have a world of
apparent motion, with bodies moving harmoniously amongst one another,
each a cross-section of some part of an unchanging and unmoving
three-dimensional entity.

Now augment the whole by an additional dimension--raise everything
one space. The helix of many helices would become four-dimensional,
and superficial space would change to solid space: each tiny circle
of intersection would become a sphere of the same diameter,
describing, instead of loops, helices. Here we would be among
familiar forms, describing familiar motions: the forms, for example,
of the earth and the moon and of their motion about the sun; of the
atom, as we imagine it, the molecule and the cell. For is not the
sphere, or ovoid, the unit form of nature; and is not the spiral
vortex its characteristic motion, from that of the nebula in the sky
to the electron in the atom? Thus, on the hypothesis that our space
is traversing four-dimensional space, and that the forms of our
space are cross-sections of four-dimensional forms, the unity and
harmony of nature would be accounted for in a remarkably simple
manner.

The above exercise of the imagination is a good preparation for the
next demand upon it. Conceive a dichotomous tree--one that always
divides into two branches--to pass through a plane. We should have,
as a plane section, a circle of changing size, which would elongate
and divide into two circles, each of which would do the same. This
reminds us of the segmentation of cell life observed under the