except a living being, then palaeontology has no foundation;
if the stratification of the rocks is not the effect of such
causes as at present produce stratification, we have no means of
judging of the duration of past time, or of the order in which
the forms of life have succeeded one another. But if these two
propositions are granted, there is no escape, as it appears to
me, from three very important conclusions. The first is that
living matter has existed upon the earth for a vast length of
time, certainly for millions of years. The second is that,
during this lapse of time, the forms of living matter have
undergone repeated changes, the effect of which has been that
the animal and vegetable population, at any period of the
earth's history, contains certain species which did not exist at
some antecedent period, and others which ceased to exist at some
subsequent period. The third is that, in the case of many groups
of mammals and some of reptiles, in which one type can be
followed through a considerable extent of geological time, the
series of different forms by which the type is represented, at
successive intervals of this time, is exactly such as it would
be, if they had been produced by the gradual modification of the
earliest forms of the series. These are facts of the history of
the earth guaranteed by as good evidence as any facts in
civil history.

Hitherto I have kept carefully clear of all the hypotheses to
which men have at various times endeavoured to fit the facts of
palaeontology, or by which they have endeavoured to connect
as many of these facts as they happened to be acquainted with.
I do not think it would be a profitable employment of our time
to discuss conceptions which doubtless have had their