ilarity in
structure or in origin, nor have they the same function and purpose.
Mountain ranges, or chains of mountains as some prefer to call them,
do not always run in certain directions, though this has sometimes
been claimed. Such claims correspond neither to reason nor to
observation. Mountains may be very much disturbed in the course of
years. Mountain peaks rise and fall somewhat. Chasms open and shut
here and there in them, and though there are those who pretend that
it is only the credulous who will accept the stories of such
happenings, there is no doubt that they have been established on
trustworthy evidence.

In the course of his observations in Italy, Stensen had seen many
mussel shells, which had been gathered from various layers of the
earth's surface. With regard to the shells themselves, he said that
there could be no doubt that they had come as the excretion of the
mantle of the mussel, and that the differences that could be noted in
them were in accordance with the varying forms of these animals. He
pointed out, however, that some of the mussel shells found in {158}
strata of rock were really mussel shells in every respect as regards
the material of which they were composed as well as their interior
structure and their external form, so that there could be no possible
question of their origin. On the other hand, a certain number of the
so-called mussel shells were not composed of the ordinary materials of
which such shells are usually made up; but had indeed only the
external form of genuine shells. Stensen considered, however, that
even these must be regarded as originating in real mussel shells, the
original substance having been later on replaced by other material. He
explained this replacement process in very much the same way that we
now suggest the explanation of various processes of petrification.
There is no doubt that in this he went far beyond his contemporaries,
and pointed out very clearly what was to be the teaching of
generations long after his own.

The same principles he applied to mussel shells, Stensen considered
must have their application also to all other portions of animal
bodies, teeth, bones, whole skeletons, and even more perishable animal
materials that might be found buried in the earth's strata. His
treatment of the question of the remains of plants was quite as
satisfactory as that of the animals. He distinguished between the
impressions of plants, t