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.

We must deal with the history, then, in the first place, as we should
deal with all other histories. The historical student knows that his
first business should be to inquire into the validity of his evidence,
and the nature of the record in which the evidence is contained, that he
may be able to form a proper estimate of the correctness of the
conclusions which have been drawn from that evidence. So, here, we must
pass, in the first place, to the consideration of a matter which may
seem foreign to the question under discussion. We must dwell upon the
nature of the records, and the credibility of the evidence they contain;
we must look to the completeness or incompleteness of those records
themselves, before we turn to that which they contain and reveal. The
question of the credibility of the history, happily for us, will not
require much consideration, for, in this history, unlike those of human