It would be well to ponder these lines of Herbert Spencer's which follow,
and which embody, possibly, the wildest vision the scientific mind has
ever achieved:

"Motion as well as Matter being fixed in quantity, it would seem that
the change in the distribution of Matter which Motion effects, coming
to a limit in whichever direction it is carried, the indestructible
Motion thereupon necessitates a reverse distribution. Apparently, the
universally-co-existent forces of attraction and repulsion, which, as
we have seen, necessitate rhythm in all minor changes throughout the
Universe, also necessitate rhythm in the totality of its
changes--produce now an immeasurable period during which the
attractive forces predominating, cause universal concentration, and
then an immeasurable period during which the repulsive forces
predominating, cause universal diffusion--alternate eras of Evolution
and Dissolution. _And thus there is suggested the conception of a
past during which there have been successive Evolutions analogous to
that which is now going on; a future during which successive other
Evolutions may go on--ever the same in principle but never the same in
concrete result_."

That is it--the most we know--alternate eras of evolution and
dissolution. In the past there have been other evolutions similar to
that one in which we live, and in the future there may be other similar
evolutions--that is all. The principle of all these evolutions remains,
but the concrete results are never twice alike. Man was not; he was; and
again he will not be. In eternity which is beyond our comprehension, the
particular evolution of that solar satellite we call the "Earth" occupied
but a slight fraction of time. And of that fraction of time man occupies