of the modern Italian tuffs which are not extinct exhibit a slight want
of correspondence when compared with those now inhabiting the
Mediterranean, some of the old ones being twice and a half as large as
the present, and that there is a numerical passage from strata
containing seventy per cent. of recent shells to those that are
altogether recent, or contain one hundred per cent. This is manifestly
indicative of a continually changing impression bringing on a
corresponding modelling. It is the proof of a slow merging into, or of a
measured assumption of, the new form--a transition, for the completion
of which probably a very long time is required. That the existing
reindeer is found in the same fluviatile deposits with an extinct
hippopotamus seemed certainly to prove that there was a condition of
things in which the co-life of those animals was possible in the same
locality, and that, as the physical causes slowly changed, the one might
be eliminated and the other might be left. That the regulating
conditions were altogether physical was obvious from such facts as that
in the bone-caves of Australia all the mammals are marsupial, and in the
pampas of South America they are allied to such forms as are indigenous,
armadilloes, sloths, etc., showing the tokens of lineage or hereditary
transmission. For still more remote times numerous instances of a
similar nature were detected; thus, throughout the whole Secondary
period, the essential characteristic was the wonderful development of
reptile life, while in the Tertiary it was the development of mammals.
But the appearance of mammals had commenced long before that of reptiles
had ceased. Indeed, the latter event is incomplete in our times; for,
though the marine Saurians have been almost entirely removed, the
fluviatile and terrestrial ones maintain themselves, though diminished
both in species and individuals. Now such an overlapping of reptiles and
mammals was altogether irreconcilable with the doctrine of a crisis or
catastrophe, and, in fact, it demonstrated the changing of organisms in
the changing of physical states.

[Sidenote: Cuvier's doctrine of permanence of species.] Cuvier
maintained the doctrine of the permanence of animal species from the
facts that the oldest known do not appear to have undergone any
modification, and that every existing one shows a resistance to change.
If his observations are restricted to periods not exceeding human
history, they may p