were thus returned to their original source: there was a constant
passage from one state of existence to another, and a returning back
again. 

Lastly, when we endeavoured to form some notion of the nature of the
forces exercised by living beings, we discovered that they--if not
capable of being subjected to the same minute analysis as the
constituents of those beings themselves--that they were correlative
with--that they were the equivalents of the forces of inorganic
nature--that they were, in the sense in which the term is now used,
convertible with them.  That was our general result. 

And now, leaving the Present, I must endeavour in the same manner to put
before you the facts that are to be discovered in the Past history of
the living world, in the past conditions of organic nature.  We have,
to-night, to deal with the facts of that history--a history involving
periods of time before which our mere human records sink into utter
insignificance--a history the variety and physical magnitude of whose
events cannot even be foreshadowed by the history of human life and
human phenomena--a history of the most varied and complex character.