experience has manifested itself. From first to last, all our
speculative successes and failures have agreed in teaching us
that the most general principles of action which prevail to-day,
and in our own corner of the universe, have always prevailed
throughout as much of the universe as is accessible to our
research. They have taught us that for the deciphering of the
past and the predicting of the future, no hypotheses are
admissible which are not based upon the actual behaviour of
things in the present. Once there was unlimited facility for
guessing as to how the solar system might have come into
existence; now the origin of the sun and planets is adequately
explained when we have unfolded all that is implied in the
processes which are still going on in the solar system. Formerly
appeals were made to all manner of violent agencies to account
for the changes which the earth's surface has undergone since our
planet began its independent career; now it is seen that the same
slow working of rain and tide, of wind and wave and frost, of
secular contraction and of earthquake pulse, which is visible
to-day, will account for the whole. It is not long since it was
supposed that a species of animals or plants could be swept away
only by some unusual catastrophe, while for the origination of
new species something called an act of "special creation" was
necessary; and as to the nature of such extraordinary events
there was endless room for guesswork; but the discovery of
natural selection was the discovery of a process, going on
perpetually under our very eyes, which must inevitably of itself
extinguish some species and bring new ones into being. In these
and countless other ways we have learned that all the rich
variety of nature is pervaded by unity of action, such as we
might expect to find if nature is the manifestation of an