laeontology, 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
which have, consciously or unconsciously, guided the researches of
palaeontologists ever since. Even that feat of palaeontology which has
so powerfully impressed the popular imagination, the reconstruction of
an extinct animal from a tooth or a bone, is based upon the simplest
imaginable application of the logic of Steno. A moment's consideration
will show, in fact, that Steno's conclusion that the glossopetrae are
sharks' teeth implies the reconstruction of an animal from its tooth. It
is equivalent to the assertion that the animal of which the glossopetrae
are relics had the form and organisation of a shark; that it had
a skull, a vertebral column, and limbs similar to those which are
characteristic of this group of fishes; that its heart, gills, and
intestines presented the peculiarities which those of all sharks
exhibit; nay, even that any hard parts which its integument containe