there was nothing left except casts of the bones, the solid material of
the skeleton having been dissolved out by percolating water. It was a
chance, in this case, that the sandstone happened to be of such a
constitution as to set, and to allow the bones to be afterward dissolved
out, leaving cavities of the exact shape of the bones. Had that
constitution been other than what it was, the bones would have been
dissolved, the layers of sandstone would have fallen together into one
mass, and not the slightest indication that the animal had existed would
have been discoverable.

I know of no more striking evidence than these facts afford, of the
caution which should be used in drawing the conclusion, from the absence
of organic remains in a deposit, that animals or plants did not exist at
the time it was formed. I believe that, with a right understanding of
the doctrine of evolution on the one hand, and a just estimation of the
importance of the imperfection of the geological record on the other,
all difficulty is removed from the kind of evidence to which I have
adverted; and that we are justified in believing that all such cases are
examples of what I have designated negative or indifferent
evidence--that is to say, they in no way directly advance the hypothesis
of evolution, but they are not to be regarded as obstacles in the way of
our belief in that doctrine.

I now pass on to the consideration of those cases which, for reasons
which I will point out to you by and by, are not to be regarded as
demonstrative of the truth of evolution, but which are such as must
exist if evolution be true, and which therefore are, upon the whole,
evidence in favour of the doctrine. If the doctrine of evolution be
true, it follows, that, however diverse the different groups of animals
and of plants may be, they must all, at one time or other, have been