the Mesozoic periods,
is it possible that the majority of other living things underwent a
"sea-change into something new and strange" all at once?

[Footnote 1: See an article in the _Saturday Review_, for 1858, on
"Chalk, Ancient and Modern."]

6. Thus far I have endeavoured to expand and to enforce by fresh
arguments, but not to modify in any important respect, the ideas
submitted to you on a former occasion. But when I come to the
propositions touching progressive modification, it appears to me, with
the help of the new light which has broken from various quarters, that
there is much ground for softening the somewhat Brutus-like severity
with which, in 1862, I dealt with a doctrine, for the truth of which I
should have been glad enough to be able to find a good foundation.
So far, indeed, as the _Invertebrata_, and the lower _Vertebrata_ are
concerned, the facts and the conclusions which are to be drawn from
them appear to me to remain what they were. For anything that, as yet,
appears to the contrary, the earliest known Marsupials may have been
as highly organized as their living congeners; the Permian lizards
show no signs of inferiority to those of the present day; the
Labyrinthodonts cannot be placed below the living Salamander and
Triton; the Devonian Ganoids are closely related to _Polypterus_ and
to _Lepidosiren_.

But when we turn to the higher _Vertebrata_, the results of recent
investigations, however we may sift and criticise them, seem to me to
leave a clear balance in favour of the doctrine of the evolution
of living forms one from another. Nevertheless, in discussing this
question, it is very necessary to discriminate carefully between the
different kinds of evidence from fossil remains which are brought
forward in favour of evolution.

Every fossil which takes an intermediate place between forms of
life already known, may be said, so far as it is intermediate, to be
evidence in favour of evolution, inasmuch as it shows a possible road
by which evolution may have taken place. But the mere discovery of
such a form does not, in itself, prove that evolution took place by
and through it, nor does it constitute more than presumptive evidence
in favour of evolution in general. Suppose A, B, C to be three
forms, while B is intermediate in structure between A and C. Then the
doctrine of evolution offers four possible alternatives. A may have
become C by way of B; or C may have become A by way of B; or A an