ained the materials constituting its substance by
a peculiar combination of matters belonging entirely to the inorganic
world; that, then, the animal was constantly appropriating the
nitrogenous matters of the plant to its own nourishment, and returning
them back to the inorganic world, in what we spoke of as its waste; and
that finally, when the animal ceased to exist, the constituents of its
body were dissolved and transmitted to that inorganic world whence they
had been at first abstracted. Thus we saw in both the blade of grass and
the horse but the same elements differently combined and arranged. We
discovered a continual circulation going on,--the plant drawing in the
elements of inorganic nature and combining them into food for the animal
creation; the animal borrowing from the plant the matter for its own
support, giving off during its life products which returned immediately
to the inorganic world; and that, eventually, the constituent materials
of the whole structure of both animals and plants 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.

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