Solidum naturaliter contento." The general course of Steno's
argument may be stated in a few words. Fossils are solid bodies
which, by some natural process, have come to be contained within
other solid bodies, namely, the rocks in which they are
embedded; and the fundamental problem of palaeontology, stated
generally, is this: "Given a body endowed with a certain shape
and produced in accordance with natural laws, to find in that
body itself the evidence of the place and manner of its
production."<1> The only way of solving this problem is by the
application of the axiom that "like effects imply like causes,"
or as Steno puts it, in reference to this particular case, that
"bodies which are altogether similar have been produced in the
same way."<2> Hence, since the glossopetrae are altogether
similar to sharks' teeth, they must have been produced by
sharklike fishes; and since many fossil shells correspond, down
to the minutest details of structure, with the shells of
existing marine or freshwater animals, they must have been
produced by similar animals; and the like reasoning is applied
by Steno to the fossil bones of vertebrated animals, whether
aquatic or terrestrial. To the obvious objection that many
fossils are not altogether similar to their living analogues,
differing in substance while agreeing in form, or being mere
hollows or impressions, the surfaces of which are figured in the
same way as those of animal or vegetable organisms, Steno
replies by pointing out the changes which take place in organic
remains embedded in the earth, and how their solid substance may
be dissolved away entirely, or replaced by mineral matter, until
nothing is left of the original but a cast, an impression, or a
mere trace of its contours. The principles of investigation thus
excellently stated and illustrated by Steno in 1669, are those