iom, that like effects imply like causes. If
there is any cause competent to produce a fossil stem, or shell, or
bone, 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 justification and even their use, but which are now obviously
incompatible with the well-ascertained truths of palaeontology. At
present these truths leave room for only two hypotheses. The first is
that, in the course of the history of the earth, innumerable species
of animals and plants have come into existence, independently of
one another, innumerable times. This, of course, implies either that
spontaneous generation on the most astounding scale, and of animals
such as horses and elephants, has been going on, as a natural process,
through all the time recorded by the fossiliferous rocks;