eriodicity--generally at a longer interval of pulsation--equally
affects the vegetal forms of life. The plant is sown, grows, flowers,
and fades.

Periodicity is to us less obvious in the inanimate world of molecular
changes; yet it is in operation even there. But it is more especially in
the natural motions of those so-called material masses which constitute
our physical environment that Periodicity most eminently prevails.
Indeed it was by astronomers that the operation of this Law was first
definitely recognised and recorded. Periodicity is the scientific name
for the Harmony of the Spheres.

The two periodic motions which most essentially affect and concern us
human beings are necessarily the two periodic motions of the globe which
we inhabit--its rotation upon its axis which gives us the alternation of
Day and Night, and its revolution round the Sun which gives us the year
with its Seasons. To the former of these, animal life seems most
directly related; to the latter, the life of the vegetal orders. It is
evident that the forms of animal life on the globe are necessarily
determined by the periodic law of the Earth's diurnal rotation. This
accounts for the alternations of waking and sleeping, working and
resting, and so forth. In like manner the more inert vitality of the
vegetable kingdom is determined by the periodic law of the Earth's
annual revolution. When fanciful speculators seek to imagine what kind
of living beings might be encountered on the other planets of our
system, they usually make calculations as to the force of gravity on the
surface of these planets and conjure up from such data the possible size
of the inhabitants, their relative strength and agility of movement,
etc. So far so good. But the first question we should ask, before
proceeding to our speculative synthesis, should rather be the length of
the planet's diurnal rotation and annual revolution periods. Certain
planets, such as Mars and Venus, have rotation periods not very
different from those of our own Earth.[14:1] Other things being equal,
therefore, a certain similarity of animal life must be supposed possible
on these planets. On the other hand, the marked difference in their
revolution period would lead us to expect a very wide divergence between
their lower forms of life, if any such there be, and our own terrestrial
vegetation. The shorter the annual period the more would the vegetal
approximate to the animal, and _vice versa_. It